Jeep CJ-8 Scrambler Brake System Upgrade and Repair

[MomoJeep]

Hi,

I'm, new here but excited to have found this website! I have owned the "Jeep Owner's Bible" for a long time now and have gotten a lot out of it! I have restored and upgraded a 1983 CJ8 Scrambler in almost all ways, but not the brakes. I use it to tow a #2000 boat, for outdoor expeditions and some off-roading.

32" Duratrac tires on R16" steel wheels. Backseat and Full hardtop. Rebuilt 4.2L with Mopar MPI and 4.0L head conversion. Flanged Axle d20 rear. AX15 trans. Twin stick d300. 2.5" spring lift / 1" body lift.

Recently I was driving at night near Moab, with 4 people in the Jeep (no boat). I come around a corner and there was a cow in the road... I nailed the brakes and they did not lock up! We came to a stop about 4' away from hitting the cow, but it didn't feel like we had adequate brakes! With larger than stock tires, and the extra weight of boat, gear and potentially 4 passengers, I want better brakes!

Currently the seal on the top of the MC is bad, and I'm losing paint on the inside of the fender. I tested the OEM Booster and it's definitely assisting the pedal, but I don't know how old or good it still is. Here's my plan/questions:

(1) It makes sense to start with the friction surfaces (and this is what most people seem to recommend). Get more stopping power without jacking up the pressure, right!? But since my MC needs to be replaced, I figure it's a good time to consider a new booster.

(2) For the front I am considering "Stoptech" Kevlar/Aramid composite pads and slotted Cryo-treated rotors. The tech guy at buybrakes.com says I can expect around 20% more stopping power in the front with this setup.

(3) For the rear I'm considering swapping the 10" oem drums up to 11" CJ drums. I believe this will create around 10% more stopping power in the rear. I'd also consider composite liners but not sure if anyone makes them.

(4) Replacing the rubber brakes hoses with braided SS.

 

Questions:

(1) Were the stock oem tires 28" or 29"?

(2) Do the 11" drums have larger capacity wheel cylinders than the 10", and if so does this require and change to the Proportioning valve or MC?

(3a) If I get 20% more stopping power in the front and 10% more in the rear, does this throw off the ratio for the safest possible braking? (3b) Will the stock Proportioning Valve be OK now that the front and rear no longer have the same ratio of stopping power relative to each other?

(4) Is there anyone who sells linings for the rear drum brakes that are high performance composite to match the front?

(5) If I upgrade the friction surfaces according to this plan, will the addition of the Dual-diaphragm booster be overkill? Would it make the brakes too touchy? (I also drive on snow and ice). 

(6) Will doubling the pressure (Dual-diaphragm) be too much for any part of the oem system?

 

[MomoJeep]

Oh, and, I'm considering switching to Silicon dot5 Synthetic fluid, mostly because it doesn't absorb water. (The Jeep spends most of it's time in Seattle area - wet climate). Is this a good idea?

(I'd vac pump all the dot3 out and possibly flush with denatured alcohol before adding the dot5.)  

[Moses Ludel]

Welcome to the forums, you have a great Jeep CJ model!

See my red highlight comments about your questions:

10 hours ago, MomoJeep said:

Questions:

(1) Were the stock oem tires 28" or 29"?

Stock tires on a 1983 CJ-8 Jeep Scrambler should be 27.1" to 27.7" for the base model (205x75R15 or 215x75R15).  Larger tires are often put on these vehicles, typically with a lift kit needed for 31" or larger sizes.  The base tire size was the basis for the OEM axle ratios.

(2) Do the 11" drums have larger capacity wheel cylinders than the 10", and if so does this require and change to the Proportioning valve or MC?

First off for this conversion, verify whether you need to change the brake backing plates as well as the drums, shoes and hardware...As for proportioning, my main concern would be piston diameter for each type wheel cylinder.  According to a quick look at Omix-ADA, the 10" and 11" rear drum brakes use the same size bore wheel cylinder, there is a left side and right side type.  You need to verify this, as the parts listings for 10" x 2", 10" x 1-3/4", 11" x 2" and so forth are very misleading.  I spent half an hour searching for these sizes and their applications without much certainty.  If the bore sizes are the same, the proportioning should be close, the difference being the swept area of brake lining material and a slight leverage effect difference between the two diameters.

(3a) If I get 20% more stopping power in the front and 10% more in the rear, does this throw off the ratio for the safest possible braking? (3b) Will the stock Proportioning Valve be OK now that the front and rear no longer have the same ratio of stopping power relative to each other?

The bigger factor here is brake bias.  The front disc brakes handle the majority of braking.  These percentages you describe have less to do with the proportioning.  The valve's more critical function is to start the rear brake application slightly ahead of the disc front brake fluid application.  This is done to compensate for the take-up time of shoes having to reach out and contact the drums.  Disc brakes have the pads adjacent to the rotors for nearly immediate take-up.  If this preset of the rear brakes did not occur, the heavier brake application at the front would dive the front end, lift the rear, and result in the rear brakes likely locking up and skidding the tires due to light rear tire contact with the road surface.  

(4) Is there anyone who sells linings for the rear drum brakes that are high performance composite to match the front?

Yes, in a sense.  Composition varies between manufacturers and within the "standard" versus "premium" or "professional grade" lining grades.  I use Raybestos when concerned about braking efficiency.  Go to the official Raybestos website and review the latest advances in braking materials.  Choice accordingly, you can gain much here.

(5) If I upgrade the friction surfaces according to this plan, will the addition of the Dual-diaphragm booster be overkill? Would it make the brakes too touchy? (I also drive on snow and ice). 

With your longer CJ-8 wheelbase, the dive and touchy issue is much less of a concern than with an 84" wheelbase CJ-5.  60Bubba has been happy with his braking upgrade, search our exchanges under "brake booster" with the search box set for "All Content".  If you like your current brake feel and booster size, that's fine, too, it's a matter of pedal pressure desired and your driving habits.

(6) Will doubling the pressure (Dual-diaphragm) be too much for any part of the oem system?

The dual-diaphragm will not "double" the brake application pressure, you can only use so much pressure to stop the vehicle.  Your concern is valid, though.  The issue I find more disconcerting would be the use of a heavy booster to compensate for poor brake sizing.  A classic example would be the vintage 9" x 1-3/4" brake shoe CJ Jeep models with a retrofit in-line Hydrovac to compensate for the small brake shoes and drum swept area.  These modifications were notorious for blowing out wheel cylinders, as the real cause of the "hard pedal" on a vintage CJ was the tiny brake surface area.  If you use OEM disc front brakes and 11" x 2" drum rear brakes with correct wheel cylinder bore sizes, the booster should not bend or blow out parts.  

A conservative approach would be changing the rear brakes to 11" x 2" if desired and choosing front and rear brake lining carefully with performance in mind.  Fresh (new or resurfaced) rotors would be advisable, these are inexpensive now in the aftermarket.  Drums should be new or resurfaced within factory recommended diameter.  Again, new drums are not expensive and new is better than used or resurfaced these days.  New wheel cylinders will help.  The stock proportioning/combination valve should work okay...Your idea of stainless braid brake hoses is a good one to eliminate pressure swelling in the hoses, although OEM hoses in good condition or new are ample for most purposes.

A new booster and master cylinder sound in order, whether a simpler 9" booster or dual-diaphragm.  For comparison purposes, Summit Racing shows this listing:  https://www.summitracing.com/search/year/1983/make/jeep/model/scrambler/department/brake-systems/part-type/master-cylinder-and-brake-booster-assemblies?N=4294950304%2B4294944837%2B4294940380%2B4294947838%2B4294928864&SortBy=Default&SortOrder=Ascending.

If your boat trailer has any weight of consequence, electric brakes on the trailer help tremendously.  A load distribution hitch with sway control (brake) is wise if the boat has much weight.  Considering the CJ-8 wheelbase, a loaded trailer at 2000 pounds or more certainly needs brakes.  (I'm not a fan of heavy trailering with any Jeep CJ.)  We can discuss this further in a new topic...See what Washington State requires...Here are some general details:

State by State Brake Laws - Karavan Trailers

www.karavantrailers.com/brakes/Brakes.xlsx

A trailer with a gross weight of 3000 lbs, GVW or more, or a gross weight that exceeds the empty weight of the towing vehicle, must be equipped with brakes that can adequately control the movement of and stop and hold the trailer. Required over 2000 lbs. GVW, On all wheels.

 

As for DOT 5 silicone brake fluid, after a denatured alcohol system flush, thorough drying and new caliper and wheel cylinder rubber, you would need to always use this type fluid—every time you top off.  I support the concept, but it's expensive; the newer formulation DOT 4 is simpler to use, more compatible and serves well.  You have a sealed, bellows master cylinder cap, which when working properly will resist drawing moisture into the system—unlike an atmospherically vented cap.  Replacing your master cylinder will provide a new bellows cap and seal.  This will dramatically reduce moisture dilution of OEM/DOT 3 "hygroscopic" brake fluid.

 

Moses

 

[MomoJeep]

Wow! Thanks for all the great answers!  And thanks for mentioning trailer brakes! That would really address a lot of my concerns. Because I launch mostly into salt water, I wanted to avoid trailer brakes. I've heard the salt water makes them a pain to maintain. However, I believe Stainless Steel trailer brakes would be a good option?

A few clarifications:

Brake Fluid: Does DOT4 offer any advantage over DOT3 other than higher boiling point (which I assume is not an issue in jeeps)?

My Questions 2&3: I see what you are saying, that activating the rear brakes just before the front in the most important function of the Combination Valve. However, I'm still concerned about the front / rear braking force ratio. Pardon me if I'm over-thinking this, but this is how my brain works... The 10% more braking I was referring to comes from going from the 10" Drum diameter to 11" (about a 10% increase). The 20% percent difference in the front is about what I've been told high performance composite pads and rotors will gain me. Now, lets say the stock setup is for 30% of the braking force coming from the rear, and 70% from the front, and the proportioning part of the combination valve is calibrated to send the right amount of pressure to the rear, and the right amount (but different) to the front, to support the 30%/70% rear/front braking force ratio, (and also because calipers and drums require different pressures). Therefore say there's Xpsi going to the rear, and Ypsi to the front. I'm assuming the front / rear braking force ratio is set by the oem designers for optimal braking(?). Now say I make these changes to the friction surfaces, such that it throws off this ratio: (1) Is it bad that the prop valve is delivering pressures to match the front and rear oem friction forces respectively, but I have altered these friction forces (in other words Xpsi to the rear and Ypsi to the front in no longer the correct psi's to match the new friction forces)? (2) My guess is that as long as the gain in the rear doesn't exceed the gain in the front, I'm ok (because the rear would not lock up first)? (3) Is it possible to get a combination valve with a different front/rear proportioning ratio that could fit my new setup? (4) And my numbers "10%" and "20%" are only estimates anyway, so I don't even know what the new ratio would really be. So do you think I should consider also adding an adjustable prop valve in addition to the oem Comb valve to dial things in better. Or will it be close enough?

OEM MC's vs aftermarket: I read somewhere that oem MC's had a check valve of some kind that maintained 10psi of residual pressure in the rear brake line. This kept the rear brakes ready to engage quickly. But aftermarket MC's don't have this feature. Is that true? If so, is it worth adding a 10psi residual pressure check valve to the rear line?

Drilled Drums: I have read some people had DIY driller small 1/8" holes in a pattern in the drums, to allow them to dry out quicker if they went into water/mud deep enough to flood into the drums. Is this a good idea? However, I don't do a lot of that kind of off-roading. But I do drive a lot on wet roads with water splashing all around. In this case I'd be worried that drilled drums would actually let splashing water get in. Agree?

 

Thanks! 

[Moses Ludel]

You're welcome, MomoJeep...See comments below:

On 5/17/2017 at 3:22 AM, MomoJeep said:

Wow! Thanks for all the great answers!  And thanks for mentioning trailer brakes! That would really address a lot of my concerns. Because I launch mostly into salt water, I wanted to avoid trailer brakes. I've heard the salt water makes them a pain to maintain. However, I believe Stainless Steel trailer brakes would be a good option?

Stainless steel would be valuable, obviously costly, but part of launching boats in saltwater...I understand the dilemma...How much does the trailer and boat weigh?  The 2000-pound concern is even more relevant with a Jeep CJ.  Your 32" tires on presumably wider than stock rims do help some with stability, at least offsetting the lift; the CJ-8's wheelbase helps, too.  Still somewhat risky business...Tow cautiously... 

A few clarifications:

Brake Fluid: Does DOT4 offer any advantage over DOT3 other than higher boiling point (which I assume is not an issue in jeeps)?

With trailering, brake fluid temp might be a concern.  Contemporary vehicles have gone to DOT 4, this is compatible with DOT 3 in a pinch.  Some fluids are now called DOT 4/DOT 3, so current formulation differences are not dramatic.

My Questions 2&3: I see what you are saying, that activating the rear brakes just before the front in the most important function of the Combination Valve. However, I'm still concerned about the front / rear braking force ratio. Pardon me if I'm over-thinking this, but this is how my brain works... The 10% more braking I was referring to comes from going from the 10" Drum diameter to 11" (about a 10% increase). The 20% percent difference in the front is about what I've been told high performance composite pads and rotors will gain me. Now, lets say the stock setup is for 30% of the braking force coming from the rear, and 70% from the front, and the proportioning part of the combination valve is calibrated to send the right amount of pressure to the rear, and the right amount (but different) to the front, to support the 30%/70% rear/front braking force ratio, (and also because calipers and drums require different pressures). Therefore say there's Xpsi going to the rear, and Ypsi to the front. I'm assuming the front / rear braking force ratio is set by the oem designers for optimal braking(?). Now say I make these changes to the friction surfaces, such that it throws off this ratio: (1) Is it bad that the prop valve is delivering pressures to match the front and rear oem friction forces respectively, but I have altered these friction forces (in other words Xpsi to the rear and Ypsi to the front in no longer the correct psi's to match the new friction forces)? (2) My guess is that as long as the gain in the rear doesn't exceed the gain in the front, I'm ok (because the rear would not lock up first)? (3) Is it possible to get a combination valve with a different front/rear proportioning ratio that could fit my new setup? (4) And my numbers "10%" and "20%" are only estimates anyway, so I don't even know what the new ratio would really be. So do you think I should consider also adding an adjustable prop valve in addition to the oem Comb valve to dial things in better. Or will it be close enough?

I understand how you might conclude that the "stronger" (if that's even so) braking would be slightly toward the front if you do your modifications.  The hydraulic apply pressures, however, are first determined by the master cylinder bore size (or sizes if the bore is stepped).  Caliper or wheel cylinder apply pressure is determined by the output pressure from the master cylinder and the bore sizes of the wheel cylinders and calipers.  If you continue to use the stock/OEM type front calipers, and if the 11" brakes do have the same wheel cylinder bore size as your current 10" brakes, hydraulic apply pressure should be approximately the same with 11" versus 10" brakes.  

The increase in lining area at the rear brakes would provide improved braking, but it is doubtful that this would be enough to throw off the "proportioning" or actual brake response.  (Of course, I would recommend testing the system on asphalt and straight-line dirt without traffic involved to be sure you're comfortable and familiar with the braking response.)  

This may be helpful:  The combination/proportioning valve for CJs in the 1982-86 range is the same, including AMC Model 20 versus Dana 44 (1986 only) equipped models.  The distinction is 1981 versus 1982-86 vehicles.  From the Mopar parts catalog:

LINES AND HOSES, BRAKE CJ-5,6,7,8, SCRAMBLER
1 VALVE, Proportioning
J5359220 1 1981
J5356622 1 1982-86

OEM MC's vs aftermarket: I read somewhere that oem MC's had a check valve of some kind that maintained 10psi of residual pressure in the rear brake line. This kept the rear brakes ready to engage quickly. But aftermarket MC's don't have this feature. Is that true? If so, is it worth adding a 10psi residual pressure check valve to the rear line?

If an aftermarket performance master cylinder is for 4-wheel disc brakes, it would not have the residual pressure valve needed for drum brakes.  For drum rear brakes, this is more of a concern than hypothesizing about the percentage of braking improvement from various lining materials or swept area.  With drum rear brakes, it is imperative that you have a residual valve in the rear brake line system.  

On your 1983 Jeep OEM master cylinder for disc front/drum rear brakes, there is a residual valve beneath the master cylinder rear brake port seat.  Disc front brakes of your Jeep's vintage do not use a residual valve to the front calipers, as this would cause brake pad drag against the rotors with the brake pedal released.  This does not occur at the rear because the the brake shoe retraction springs have strong enough pull to overcome the hydraulic residual pressure (typically 12 psi or so, maybe less if your source is correct.) 

The purpose of residual pressure with wheel cylinders (drum type) is to hold the rubber cup lips flared outward when the brake pressure/pedal is released.  If the lips collapse from no residual pressure in the system, brake fluid could leak past the rubber cups and moisture could seep into the cylinder.  By contrast, a disc caliper piston with a rubber square-ring will seal the piston and bore of the caliper without the need for residual pressure.

Note: Some later disc braking systems do use very slight residual pressure to assure constant pad alignment with the rotor.  Pressure is low enough to prevent drag and premature pad wear.  This is not the kind of pressure applied with older drum brake wheel cylinder residual valves.

Drilled Drums: I have read some people had DIY driller small 1/8" holes in a pattern in the drums, to allow them to dry out quicker if they went into water/mud deep enough to flood into the drums. Is this a good idea? However, I don't do a lot of that kind of off-roading. But I do drive a lot on wet roads with water splashing all around. In this case I'd be worried that drilled drums would actually let splashing water get in. Agree?

I personally would not drill holes to relieve stream water, as this would also allow debris and dust to enter the drum.  Abrasive trail dust would be rough on brake lining, drum surfaces and rubber parts.  Arguably, if the vehicle is in and out of streams continuously (a trailer queen 4x4 driven through water crossings regularly), maybe some of that dust would be washed out.

Without drilling holes, there is a gap between the drum and backing plate plus open areas at the brake shoe edges that allow water to exit the drum enclosure.  This may not be as quickly as drilled holes, but it would be consistent with OEM design.  If I were driving through streams or water fording regularly, my approach would be four-wheel disc brakes.

Moses

Thanks! 

 

[MomoJeep]

Moses,

Thank you so much! This is all very helpful and I feel ready to start by replacing my MC (the lid seal is leaking) and figure I might as well try replacing the single diaphragm booster with a new double-diaphragm while the MC is being changed.

Lastly, from this conversation I'm now confident the oem Combination valve is fine, however, what do you think of replacing it. I do not know if it's the original one, but I think it probably is. I bought this Scrambler about 15 years ago as a pile of parts (some else's rebuild project that they gave up on). I took about 10 years to slowly do a frame-up restoration, and have been driving it for the last 5. So that's the history of the current Combination valve. Do they tend to last forever, or might I be wise to also replace it with a new one while I'm doing the MC?

Thanks!

[MomoJeep]

Any recommendations on where to purchase MC and dd-booster? I see "Restoration Performance" mention in your other thread...

[MomoJeep]

Hello again!

At this point, I feel like I'm asking too many questions, but thought this one was worth bringing up. Brake line fittings that seize and then the nuts strip. So common, so frustrating. To possible fixes aside from re-doing the lines with SS lines:

 

(1) Carefully use ant-seize on the threads. But is it not worth risking possibly contaminating the brake fluid, either now or at some point in the future when I may have forgotten I used it and I'm replacing something..

(2) Stop over-tightening them. Apparently they don't have to be super tight, and if they weren't, they wouldn't seize. Make em' snug, then check for any leaks. If s small leak, make 'em a tad snugger.

Which is the better plan?

Thanks, and I super appreciate any responses!

 

 

 

 

 

[Moses Ludel]

20 hours ago, MomoJeep said:

Hello again!

At this point, I feel like I'm asking too many questions, but thought this one was worth bringing up. Brake line fittings that seize and then the nuts strip. So common, so frustrating. To possible fixes aside from re-doing the lines with SS lines:

 

(1) Carefully use ant-seize on the threads. But is it not worth risking possibly contaminating the brake fluid, either now or at some point in the future when I may have forgotten I used it and I'm replacing something..

I would not use anti-seize for the reasons you cite.  Any petroleum/mineral base in the anti-seize compound would be damaging to rubber brake parts/seals/cups.  

Even a flare nut wrench offers no guarantee that the wrench will not spread under pressure and round the flare nut's corners.  My time-honored solution has been a unique Wheeler Manufacturing Company chain wrench gifted to me by a plumbing sales rep in 1969.  The chain wrench will grip the flats of the flare nut while not spreading under pressure.  This does not damage the flare nut yet grips and can rotate with any torque needed.  I have used the tool on 3/8ths inch size flare nuts and vintage British motorcycle fork caps of 1-1/2" hex size.  Oil filter canister base removal has also been done with this tool.

Here is an example of my chain wrench, a classic tool that can be used for loosening and tightening as necessary (when it is desirable to reuse a tube and flare nut if the flare nut flats are questionable).  When using a torque wrench for precise tightening, of course, you need a flare nut that is completely intact and a flare nut crow's foot:

You'd be fortunate to find one of these wrenches...It can grip very well and apply substantial torque when desired.

 

 

Quote

(2) Stop over-tightening them. Apparently they don't have to be super tight, and if they weren't, they wouldn't seize. Make em' snug, then check for any leaks. If s small leak, make 'em a tad snugger.

They do need to be tight enough for the tubing flare end to seat and conform to the wheel cylinder seat and seal properly.  If over-tightened, however, the double-flared tubing end can split.  Make a point of applying torque that is equivalent to the tension described in the FSM for your Jeep.

Moses

 

[Moses Ludel]

As for replacing the combination valve, these units last a very long time unless there is corrosion or contamination of the braking system.  If the valve seems to work okay, it likely is okay.

Make sure you follow the J-tool procedure when bleeding the brake system on your Jeep.  If the combination valve is not held open, you will have a very difficult time (if not impossible) bleeding the brakes front-to-rear.  Note the tool described in the FSM or simulate its function.

Regarding sources for a performance booster, I'm not familiar firsthand with any of the current suppliers.  That's why I mentioned 60Bubba and his account.

[MomoJeep]

Well I installed the 8" dual diaphragm and honestly I don't notice much difference over my oem single (9" I think, it measures about 8-3/4" dia). When I first installed it, it almost felt like no booster at all. Then I noticed the plastic vacuum hose fitting seemed to be defective, it was hard for me to suck air through it with my mouth. So I replaced it, but also noticed that they are using check valves. My oem did not seem to be a check valve..

(a)  should I not really expect any noticeable difference in braking power with the 8" dual over my oem single?

(b) Why would they be using a check valve? Shouldn't it just be a through fitting?

[MomoJeep]

Also, I had to pull the brake lines forward about an inch with the Dual-Diaphragm setup. No coils. but they come up the firewall and turn 90deg and head to the MC. I also have a 1" body lift, so after the 90deg bend, they now angle upwards to reach an inch higher. I was afraid I'd need to get new longer lines, but there didn't seem to be any real added strain on the lines. They simply pulled forward away from the firewall a bit. Do you think this is fine, or should I worry about replacing them?

 

[Moses Ludel]

Assuming that the dual-diaphragm booster would work as well as others describe, it sounds like you need to follow through with other concerns in the braking system, which we've discussed within this topic.  Simply applying more pressure (if that's what this new booster does) may not offset rotor, drum, pad, shoe or caliper/wheel cylinder issues. Also, are you bleeding the system according to the factory procedure?  If not, or if the combination valve has switched over to feed only one end of the CJ's braking system, you may discover that the brake system operates at one end of the vehicle only. When testing the brakes with the vehicle off the ground, are you able to lock up all four wheels when the brakes are applied?

The purpose for coils in the brake pipes below the master cylinder is to allow for any movement between the body (the mounting point of the booster, master cylinder and pedal) and the separate frame or "drivable chassis" of your CJ-8/Scrambler. There is flex, especially off-pavement, and movement of the body above the frame. The brake tubing goes from the master cylinder to the rigid attachment points at the frame's brake fittings or a "T".

In the day, firewall mounted master cylinders often used a brake hose from the master cylinder to the frame tubing. Later single and dual braking systems shifted to the coiled tubing approach, which offers enough "spring" to absorb the movement and flex between the vehicle's body and a separate frame. (This would be less likely with a uni-body like the XJ Cherokee, though the practice of coiling the tubing is used on these models, too.)  The concern is two-fold in your case: 1) whether there is enough room for the frame and body flexing and 2) if the tubing is too rigid, it is at risk of fracturing from movement and vibration over time.

I emulate OEM brake system approaches.  If your CJ-8's tubing was originally coiled, I would coil it. For more ideas, there are endless examples of coiled brake tubing below the master cylinders on modern vehicles.

As for braking performance, if the new booster is working properly, check out the rest of your brake system. Regarding the booster check valve, this is usually a one-way valve. The OE style valve traps air in the booster so that there is always vacuum assist and a quick recovery, regardless of the engine's manifold vacuum at a given time. By design, the check valve will hold vacuum in the booster when the engine shuts off (not for an indefinite period) to provide at least a couple of brake applications with power assist in case the engine stalls. On a Jeep 4x4 used on- and off-road, this is crucial.

If you eliminate the one-way feature of the valve, you would have erratic vacuum dependent on the engine running and producing sufficient, stable vacuum. Trapping vacuum in the booster with a one-way check valve assures that the booster has adequate vacuum at all times...Make sure you're using the check valve designed for the booster, a check valve that provides one-way hold of the vacuum if required. Check the valve's suction in both directions. A one-way valve will suck/move air in one direction only. It holds vacuum in the booster reservoir. The check valve should not allow vacuum to leak off in the opposite direction. When testing, this means the air moves only one way.

[MomoJeep]

Thanks!

MC brake lines: Yes, the oem setup is (not coiled) straight up the firewall, then 90deg bend to the MC. The 90deg bend seems to serve the same purpose as the coils, because the lines are now in an "L" shape, so there is room for movement as compared to if they went straight to the MC.

Booster: yes, I checked my combination valve. I checked by cracking open the front bleeders and stepping on the brakes. I heard it "pop" and the brake warning light came on. Then I bled the brakes with the combo valve "button" held in the pushed in position, and it seems to have reset because the warning light went off. But I have not checked the brakes by having the vehicle off the ground.

I also replaced the MC (bench bled). And I know my front brakes (oem setup) are in good shape (just replaced them last year). However, I have not inspected the rear (I replaced them a few years ago).

I know the booster is "doing something" because if I put my foot on the brakes and then start the engine, I feel the boost in the pedal, just like I used to feel with the single diaphragm. But I still don't understand why the dual-diaphragm doesn't feel noticeably different than the single in terms of stopping power..

 

[Moses Ludel]

Did you confirm the check valve's function and whether you have vacuum in the booster after the engine shuts off? Easy test:  After shutting off the engine, you should have a couple of power assisted brake applications before the captured booster vacuum is evacuated.  If not, you do not have a functioning one-way check valve.

The off-the-ground brake test should be helpful.  Also curious what you find after removing the rear brake drums.

[MomoJeep]

After shutting of the engine, if felt like I got one power assist, but not two. Then put my vacuum tester inline between the vacuum hose and the fitting on the booster, and I get 16-17psi. I also noticed that after shutting off the engine, If I pop the booster fitting off, it goes "Whoosh" and sucks in a bunch of air. So it seems to be working.

When I slam on the brakes, none of the tires will lock up, and the jeep doesn't exactly lurch to a stop. Apparently the front tires should lock up first by design. But since they're not, I figured I'll add friction there. I've order some front composite brake pads that claim to be about 20% more friction. We'll see how that goes.

I'll also pull the rear drums off, and do the off-the-ground test..

I was wanting to go from 10" rear drums to 11", but I've been told the 11" backing place are almost impossible to find for my amc20 rear axle. They only used the 11" on '76-'77 CJ's...  Apparently the oem 10" drums are pretty good, but the oem from disks are more like "car brakes".

[Moses Ludel]

Under normal and hard braking, the front brakes do catch the brunt of the braking effort. This is normal for any drum, four-wheel disc or disc/drum braking system.  When brakes are applied, the vehicle's weight transfers to the front brakes.

So, a front pad composition change can help as long as the rotors and calipers are in good condition.  I'm suspect of the rear drum brakes' condition.  Look for glazed or even cracked rear lining, glazed or out-of-round drums, and check for wheel cylinder seal leaks and binding pistons.  The simple off-the-ground brake test with the drums still in place could be helpful.  Inspection with the drums removed should be conclusive.

Note: You can remove the rear brake drums without disturbing the wheel hubs. You want to avoid hub removal unless you are performing axle shaft, shaft bearing or axle seal work.  Positioning the wheel hubs properly and re-torquing the axle shaft hub nuts to factory specification is a critical chore.  The factory recommendation is to install a new hub/flange every time you remove a rear hub on the Jeep CJ's AMC 20. (See my Jeep CJ Rebuilder's Manual: 1972-86 for service details on the rear hubs and axle shafts.)

The OEM 10" brakes work adequately in any case if the brake parts are in good condition, and this includes the drum surfaces, brake lining/composition and the wheel cylinders.  Older wheel cylinder pistons can bind and not apply full brake pressure.  Also check to be sure the rear shoes are installed correctly:  The longer brake lining faces toward the rear of the vehicle, the shorter brake lining faces toward the front of the vehicle.  These are "self-energizing" brakes with an anchor pin. If the lining lengths are reversed, the braking effectiveness will diminish dramatically when the vehicle is moving forward and the brakes are applied.

Moses

[MomoJeep]

Well thank you for suggesting the wheels off the ground test. When I stomp on the brakes, the Jeep does lurch forward, so I figure the front brakes ARE working. So I jacked up the rear (I'm also more suspicious of the rear) and put it in gear. With the rear wheels both turning, I noticed:

 

(a) Braking does not engage right away as I put pressure on the pedal. I would have expected them to engage right away, but they didn't.

(b) With more pedal pressure, I killed the engine, and also heard a loud snapping sound.

(c) When I tried again, I could get one rear wheel to lock up, or the other, but not both, and the engine would lug a little, but not die! Brake pedal fully depressed, one or the other of the rear wheels still turning...!

If just one of them locked up consistently, I'd suspect that specific wheel to have a problem. But since that's not the case, my best guess is that there is air in the rear line somewhere. I also noticed the rear reservoir in the MC has lost some fluid. I don't see any obvious leaks at the MC or at either of the rear wheels, so I'll check the lines underneath...

[MomoJeep]

Update:

After removing both rear drums, I found a LOT of black dusty powder in each of them. I also notices that the rear shoe on the driver side is cracked in the middle, and on the passenger side some of the rivets are no longer in their original location relative to the lining, as if the lining has "pushed past" them, stretching the original holes out-of-round.

THEN, I noticed the two short shoes are on the driver's side, and the two long ones on the passenger side. This may explain the inconsistency in which rear wheel would lock up.

Other than this everything else looks in order. I check for a fluid leak from the MC all the way through the Combo valve to each rear wheel, and I do not see any obvious leak. The inability to lock up both rear tires and kill the engine (wheels off the ground) could be explained by the mismatched shoes rather than air in the line, but I still don't understand where the fluid went. I just put in a new MC about a week ago, and filled it to within 1/4" of the top, Now the Front is still properly filled, but the rear has dropped about 1/4". Any idea where than fluid went!? (maybe I just forgot to top it off after final bleeding - but I that would be an odd mistake for me to make...) ??

While I'm in there, I'm thinking of replacing everything: wheel cylinders, shoes, and hardware... Do you think that's a good idea?

[MomoJeep]

and for clarity sake: The metal frames of the brake shoes are all the same length. It's just the linings on the shoes are either around 8" or 10".

[MomoJeep]

The wheel cylinders appear to be fine. I do not know how to test them or otherwise determine if they are binding or otherwise not working well. They are about 5 years old with low mileage on them (not a daily driver -maybe 30k miles). Would you suggest replacing them with new ones, or going with the "if it's not broken, don't fix" philosophy? Honestly, my concern is that I'll install new ones and then one of them will leak. For some reason, my experience with hydraulic parts is that they either leak soon after  being installed, or they last a long long time. Any idea why they sometimes leak when new? Would you suggest replacing or leaving alone?

The drums don't look bad. The friction surfaces are a little shinier than the new ones, but not exactly "glazed". I'm thinking of replacing them as well..

[Moses Ludel]

If the wheel cylinders are not leaking and you know their service life is relatively low, you can concentrate on replacing the shoes, installing new hardware kits and resurfacing the brake drums.  (Check the drums for round and whether they can still be resurfaced within legal diameter.)  You found plenty wrong with the rear brakes, enough to account for very poor braking.

Restore the rear brakes and assemble the shoes properly and in sequence.  This should change the function of the rear brakes dramatically and will likely be your overall brake "cure".  The 1/4" drop in fluid is probably from air in the rear system that has vanished or from the rear brakes continually self-adjusting for wear.  The shoes are dangerously defective.

[MomoJeep]

Yes, thank you. One of the wheel cylinders had lots of fluid behind both piston caps, maybe they were extending too far. The other one seemed have corrosion around the outside of both pistons, as if they were NOT extending as far as they should. This might makes sense considering two long shoes on one side, two short ones on the other. So I decided to replace the wheel cylinders as well. Thanks for your help on this. I'm looking forward to seeing how the brakes feel once I put it all back together correctly, with all brand new parts!

 

[Moses Ludel]

MomoJeep...Fluid between the cylinder pistons and the dust caps is not a good sign.  The fluid has been seeping past the rubber sealing cups.  The cost of new cylinders is very low these days.  Buy a couple of quality new rear wheel cylinders during this major brake job.  You need the rear hydraulic system to hold pressure.

Your rear brakes have been troubled for some time.  Restoring the rear brake function should make a major difference in overall braking performance.  Drum brake shoes at the rear generally last longer than front disc brake pads.  For that reason and the effort to remove the drums for brake inspection, the rear brakes often get neglected.  Adding the booster might have been the catalyst for the leaking rear wheel cylinders, they were stressed/worn and likely in the process of failing.  Now you know the booster is doing its job.  With new rear wheel cylinders, brake shoes, hardware renewal and fresh drum surfaces, you'll have superior braking.

The glazed and cracked brake lining sounds like inadequate lining or brake overheating.  We began this discussion with thoughts about trailer weight and stopping without trailer brakes.  That kind of heat, especially when slowing the vehicle on downgrades with a trailer load, will cause quick deterioration of brake components.

[MomoJeep]

Yes, thanks. I replaced everything. I also had to replace a brake line and in the process let the MC run dry. I'm assuming this means I should pull it out and bench bleed it?

[Moses Ludel]

You can fill and bleed the master cylinder in the chassis.  With the brake pedal released, fill the master cylinder reservoir and let the fluid run into the bore cylinder's bore.  From there, it's the usual bleeding job; on your CJ-8 Scrambler, just remember to hold out the stem on the combination valve during the bleeding job to keep the front and rear brake systems open.  Verify that you can bleed each end of the vehicle.  When you finish, make sure that the brakes are operative at both ends of the Jeep...This means the combination/switch-over valve is in the balanced (brakes working at each end) position.

I like to use a one-person filler atop the master cylinder while priming and bleeding.  This prevents the master cylinder from going dry during the bleeding process.  Here's the OTC 8106 device, there are similar types in the market for other cap designs.  I also use a vacuum bleeder.  In combination, this is a great solution for the one-person bleeding job:

https://www.otctools.com/products/master-cylinder-auto-filler

 

 A vacuum bleeder is a better approach, as it will suck the debris from the corners of the wheel cylinders and caliper.  By contrast, pressure from the master cylinder can push debris to the edges of cylinders, away from the bleeder valve.  Vacuum at the bleeder valve is a good way to purge old fluid periodically, something most techs and shops overlook.  

Many brake systems never see a flush or fluid exchange between overhauls.  This is not a good thing, as brake fluid is hygroscopic and draws an average of 3% moisture per year into the system; this lowers the boiling point and creates a corrosive environment within the brake hydraulic system.

 

 

[MomoJeep]

Great, thanks! My combination valve (1983 - I heard this was a year where they switched types in mid-year) seems be the kind with the push-in button. So I made a tool to hold it in out of a stout wire. Everything went well, except I forgot to turn the star wheel rear adjusters up to 3/8' threads showing as the repair manual says to do. I had maybe 1/16" of threads showing. I tried backing up and stopping over 15 times, then realized I could open the adjuster access hole and turn in manually. It had already adjusted a fair amount (about 1/4"). I turned it until I felt some resistance.

Do you think not having started with 3/8" thread showing caused any problems? I'm afraid this could have caused the wheel cylinder piston to over-extend?

[Moses Ludel]

Normally, with new (thicker) lining your brake cylinder pistons should be fairly far into the cylinders.  The shoes are retracted with the brake springs, it's highly doubtful that you hyper-extended the pistons, especially considering the ratio of movement at the top of the shoes (wheel cylinder position below the anchor pin) versus the movement at the adjuster (at the bottom between the shoes).  All the while, the drum holds and contains the shoes in a concentric position, which also limits piston movement.  To be on the safe side, check the brake fluid level after this degree of adjustment.

When adjusting the brakes manually at the adjuster wheel, I like to seat the shoes (tighten) cold until the drum will not rotate then back off the adjuster until the drum turns without any drag whatsoever and no sound of the shoes contacting or raking on the drum.  Properly working self-adjusters can easily take up adjustment from there.  Overall, you do not want brake drag, which would heat up lining and drums and cause premature wear.

There's a "break-in" for brake lining, which most consumers and shops overlook.  If you bought a name-brand lining, go to the website and see if there are break-in instructions.  If not, go to the Raybestos site or read this Federal Mogul quick sheet:  http://www.federalmogulmp.com/en-US/Technical/Documents/Brake Pad and Shoe Break-In Procedure.pdf.

The 20 stop procedure should be plenty, but you also need to avoid overheating the brake lining during initial use.  On that subject, have you considered a boat trailer brake solution?  You now know that the added load can adversely affect the brakes, which included the drum rear system in addition to front pads and rotors.  If you'd like to open a topic on trailer brakes, I'd be glad to respond...

Moses

[MomoJeep]

Yes, thank you, I checked the fluid and the levels did not drop.

Great tip on how to adjust the brakes!

You're right. I was not aware of the lining break-in procedure..

Yes, for the boat trailer I'm considering trailer brakes, as you suggested. To upgrade my CJ brakes and further seems like too much work and complexity.

 

Thanks so much for all your help!!

-James (MomoJeep)

[Moses Ludel]

James...This is smart.  The stock Jeep CJ-8 braking works well and should suffice, especially with the newly restored system and added dual-diaphragm booster.  Trailer brakes are the way to go, we can discuss that prospect when you're ready!

Let us know how your new brakes work...

Moses

[MomoJeep]

Thanks for all your help. I just did a long road trip in my Scrambler, 3 people and lots of camping/adventure gear. Although I still wish it could "stop on a dime" the way little economy cars do, the brakes performed well in a variety of conditions. My next upgrade will be hi-performance slotted (not through) front disks and composite pads for maximum stopping power.

 

About trailer brakes, what would you advise for a 16' boat trailer (gross weight 1800-2000lbs)?

 

[Moses Ludel]

Glad this is working, MomoJeep.  As for trailer brakes, the two kinds common for a trailer in this weight would be a surge (hydraulic, tongue mounted like the U-Haul trailers use) or an electrical/electronic controller with a seven-pin connection.  The controller mounts at the dash.

You can adjust most electronic controllers for light pressure or even off when the trailer is empty.  The weight (gross) is not considerable, and presumably this is a single axle trailer in that weight range? 

An electronic controller installed and adjusted properly would likely serve best.  The surge brake requires more adjustment effort, and it cannot be readjusted from the driver's seat...Do you know what capacity or size brakes are now on the trailer?  This trailer does have factory brakes at the axle?

Moses