1981 Jeep CJ-7 Frame-Off Build Needs Brake Upgrade Solutions

[spdljohn]

This is my first Jeep restoration  project, a 1981 CJ-7.  Going well so far.  Down to bare frame.  Got a 6.0L LS engine and ax15 tranny from Novak Conversions coming in the next week or two.

 

Ordered new axles (wide track) from East Coast gearing in Raleigh NC with disc brakes front and back.  The front brakes are Chevy 1/2 ton caliper, rear is similar to 2000 Jeep TJ, Grand Cherokee, or Ford Explorer with internal brake.  The brakes are from Centric (not sure what model #).

 

Questions:  1) What master cylinder, booster, brake line size, etc do I need to go with? 2) Do I need adjustable proportioning valve or no prop valve with just distribution box?

 

Any advice would be extremely welcomed and helpful.  Thanks in advance.

 

David J

[spdljohn]

[Moses Ludel]

Fantastic photos, David!  Shop looks great, reminds me of our rural home and shop at Yerington, Nevada.  We lived and worked from that property for 15 years (1994-2009), and the shop was backdrop for several books, including the Jeep CJ Rebuilder's Manuals!  I like your John Deere tractor, too.  My hydraulic "engine lift" was a compact John Deere 4310 diesel 4x4 with front end loader and scraper box...Smooth control there, great alternative to a cherry picker!

 

My approach with upgrade brake systems and brake master cylinders, boosters and proportioning valves is to model after a similar OEM system.  For example, if your entire braking system will be Chevy 1/2-ton caliper, I would consider a master cylinder that has dimensions like the OE donor or facsimile truck.  When I refer to dimensions, I mean two critical things: 1) piston or bore diameter and 2) stroke of the piston.  The master cylinder must displace the correct amount of brake fluid for the size of the calipers at the wheels.  The booster must be a match for the master cylinder, and the linkage from the pedal into the booster, and the booster into the master cylinder, must have proper settings. 

 

There are aftermarket master cylinder/booster combinations available that also meet these requirements and will specifically fit your Jeep chassis.  We discuss this at a forum post by 60Bubba, and his approach for a CJ-7 like yours looks practical:  http://forums.4wdmechanix.com/topic/437-dual-diaphragm-brake-booster-upgrade-for-a-cj-7-jeep/?hl=%2Bbrake+%2Bbooster.  60Bubba shares that the setup fits well.  Read down the thread, there are back and forth comments and sharing of important points.  See my comments about setting up pedal linkage free play and booster pushrod clearance.  Make sure the piston fluid displacement works with 4-wheel disc brakes and calipers like you have planned.  You need a master cylinder capable of handling front and rear disc brake calipers, not just two calipers (front) and two drum brake (rear) wheel cylinders like a stock 1981 CJ-7.

 

There are three other topic posts that provide additional insight: 1) http://forums.4wdmechanix.com/topic/216-dana-44-front-axle-swap-into-1985-jeep-cj-7/?hl=%2Bbrake+%2Bbooster#entry2188; 2) http://forums.4wdmechanix.com/topic/396-changing-1985-jeep-cj-7-from-manual-to-power-brakes/?hl=%2Bbrake+%2Bbooster#entry2077 and 3) http://forums.4wdmechanix.com/topic/219-another-grand-wagoneer-to-late-cj-parts-mate-up/?hl=%2Bbrake+%2Bbooster.

 

Note:  I accessed these four posts quickly by using the forums Search box.  First, set the search box drop down choices to "Forums".  Insert these words: "brake booster".  A similar search under the words "master cylinder" would turn up material.  The forums have grown considerably, and it's now helpful and practical to use the Search box.  With the right keywords, it works!

 

As for brake line size, again look to the size used by a GM 1/2-ton for a baseline.  (Determine whether the calipers are U.S. or metric size fittings, as this could dictate the replacement lines needed.)  The 1981 CJ-7 piping is ample and may work.  Your concern here is sensible, though most modern disc front, drum rear brake systems have sufficient brake tubing size.  Compare to be sure, and if you can keep the OE tubing, it does have the advantage of armor guard or wire protective wrapping at vulnerable points.  That's a plus for a 4x4 used off-road.  If you replace piping, use brake grade tubing, wire wrapped at least for key areas.

 

For proportioning, the master cylinder/booster source may have suggestions.  There's likely a proportioning or combination valve that is optimal for the four-wheel disc brakes.  You could consider an OE setup like the later TJ Wrangler with four-wheel disc brakes to have the combination valve feature.  This provides proportioning plus a safety switch-over and lamp cue for a brake system failure at one end, similar to what your OE CJ-7 has now.

 

I have used manual proportioning valves, and they can work.  Wilwood and others offer quality products.  The simpler manual proportioning valves lack one key feature:  They will not apply "timed" brake application at one end first.  This is especially important on disc front/drum rear brakes, where the rear brakes need to receive fluid pressure just slightly ahead of the front discs.  This accounts for rear brake shoe return spring tension, shoe-to-drum clearance and lag time when compared to front disc pads that ride directly on the rotors.  Disc brakes apply immediately when fluid pressure reaches the caliper.  

 

By applying rear brakes slightly ahead of the front brakes, the timed rear braking helps prevent front end dive during heavy brake application.  This is a key function of OE proportioning on disc front/drum rear brakes.  Timing is dictated by chassis engineering, brake capacity and type, vehicle weight, weight distribution and wheelbase length, dive tendency and risk of wheel lock-up. 

 

Rear brake lock-up is always a concern, and the system must be balanced to prevent the rear wheels from locking up under non-skid braking.  When only the rear wheels lock up, the most common cause is front end dive and weight pitching forward.  When this happens, the rear of the vehicle lifts up and lightens the load on the rear tires.  Rear tire/wheel lockup occurs.

 

Note: A shorter 94-inch wheelbase vehicle like your Jeep CJ-7 poses less risk of front end dive.  Likely you have stiff springs planned and oversized tires, and this reduces front end dive even more.  Always test brakes in a safe driving environment.  Be aware that rear wheel lockup can spin a vehicle around...Your parts choices and success story here will be of great interest to others, David!

 

The simpler manual proportioning valve meters down or restricts fluid (like closing a faucet slightly) at one end of the vehicle, typically the rear.  This helps prevent wheel lockup by lowering overall fluid volume and pressure available at that end of the braking system.  Check out the Wilwood catalog, there may be more sophisticated manual proportioning setups available.  Summit Racing offers a variety of braking solutions for custom systems like you're building. 

 

My safety benchmark would be an OEM four-wheel disc brake proportioning/combination valve setup similar to either a late Jeep TJ Wrangler or a Chevy 1/2-ton truck, depending upon the master cylinder used.  Again, try the aftermarket master cylinder/booster supplier that 60Bubba used.  Explain your axle choices and wheel brake sizing.  They may have a quick, tested solution here.  Please share your findings...

 

If you decide to test the stock CJ-7 combination valve, be aware of the quirky shuttle pin at the valve when bleeding the brake system.  I talk about this in the Jeep CJ Rebuilder's Manual: 1972-86.  If you do not hold the pin in the proper position while bleeding the brakes, one end of the system will be shut off and not bleed properly.

 

Please ask questions and share your findings as the project unfolds.  Others will benefit from your "parts list" for this brake upgrade.  The CJ-7 will be an awesome machine, your photos already say a lot!

 

Moses

[spdljohn]

Moses

 

Thank you for your very informative and insightful reply.  That is very good information to have.  My axles will be ready for pick up in another couple of weeks.  Once I get then and can look at exactly what they have installed in terms of the brakes then I am planning on really diving into the brake issue.  I will definitely be documenting with photos and posting here as I go and will let you know how the system works.

Moses, can you explain the difference between a proportional valve and a combination valve?

 

Very excited about this project.  

 

David

[Moses Ludel]

David, I'm glad this is useful...Regarding the explanation of a combination valve, I'll begin with a brief anecdote.

 

I've always been "academic", which comes as no surprise to the forum members!  In the early '80s, I was teaching Automotive Technology at the San Diego Job Corps facility, and on our shoestring budget, we bought a used 1972 Buick Riviera.  (The 455 V-8  was emission detuned that year with lower compression, yet the torque was still impressive.)  I highly value automotive and truck OEM and professional trade manuals, and to my great satisfaction, this one-owner car came with a factory workshop manual in the trunk.

 

Of course, I used the book faithfully for service work, and the early '70s Buicks did not require much beyond routine maintenance.  (The largest task was removal of the cylinder heads for a valve job.)  That book was packed with wisdom pearls, as GM had a wonderful habit of explaining the function of components, not just how to replace them. 

 

The book shares details on the early combination valve design and purpose.  I've applied this useful information as an adult education instructor, journalist and author (including my Jeep, Ford, GM and Toyota truck books). 

 

It seems only fitting to share GM's insightful information as provided to dealerships and the professional trade.  Here is the official definition with a cutaway schematic depicting a 1970s circa GM combination valve.  (Zoom-in for detail.):

 

GM Description of Combination Valve.pdf

 

Note the comments on the combination valve's "Hold-Off" function to accommodate rear drum brakes that have brake shoes with return springs.  This description covers vehicles like your factory CJ-7 brakes.  With four-wheel disc brakes, the dynamic changes.  The brakes at all four wheels have disc calipers, pads and rotors.

 

Also catch the GM comment about the elimination of the proportioning function on vehicles with high rear wheel loads.  This also applies to a vehicle that has less tendency to pitch forward under hard braking.  Rear wheel lock-up occurs most often when the weight on the rear wheels becomes light from the vehicle's weight pitching forward.  A shorter wheelbase Jeep CJ-7 with wide axles, stiff leaf springs all around, likely gas charged shocks, and weighty oversized tires has less tendency to pitch forward under braking.

 

Some of us have worked with GM trucks that use a factory manual proportioning valve at the rear axle.  We had mid-'80s Suburban 3/4-ton 4x4s with this mechanical linkage and proportioning valve at the rear axle.  As the vehicle weight pitches forward under hard braking, the rear axle-to-frame distance increases.  Mechanical linkage rotates a valve at the rear axle's hydraulic system.  This decreases hydraulic pressure to the rear drum brakes to help prevent rear tire/wheel lockup.  When the truck has a trailer with a load distribution/equalizer hitch weighting the rear of the vehicle, the frame height is more level, allowing full brake pressure application at the rear brakes...Think of this as a factory "mechanical linkage proportioning valve"!

 

In the PDF illustration, note that GM uses a pin at the front of the combination valve to permit proper bleeding of the brake system.  On this GM combination valve, you depress the pin to bleed the front brakes...Similarly, on your Jeep CJ-7 OEM system, there is a special tool that holds a shuttle valve on the combination valve in a specific position during brake bleeding.  There are differences between the GM valve and Jeep type.

 

Moses

[spdljohn]

Thanks Moses. So it's called a combination valve because it does two things; provides pressure to rear brakes before front brakes, and secondly limits ultimate pressure to the rear brakes to prevent lockup?

I think I've got my plans underway for my brakes. I have spoken with Matt sweeting in California and have decided to go with a hydro boost system. I hope I'm not just buying into the hype. It it sounds really good. They said they would not recommend putting in any proportioning valve initially as chances are the rear wheels will not likely lock up with 33 inch tires.

They really seem to know what they're talking about with their systems. They also said brake line size doesn't really make any difference and that 3/16" all around would be fine.

I'll soon post pics.

David

[Moses Ludel]

David...When you look at the combination valve illustrated in the PDF (lower page), there are three functions:  1) Hold-Off, 2) Warning Switch and 3) Proportioning.  Again, this brake system is a disc front with drum rear brakes.

 

The warning switch provides the lamp signal if one end of the brake hydraulic system or the other should fail.  If your new master cylinder does not have a common fluid reservoir and is a divided cylinder with separate fluid reservoirs for front and rear brakes, there will be individual fluid supplies to the front brakes and the rear brakes.  This provides a safety factor if one end of the brake system fails or leaks.  You would not have the failure/warning switch and a lamp signal, though.  There may be aftermarket switches available for warning lamps.  One switch would be required for the front system, one for the rear.

 

I would agree that a short wheelbase Jeep CJ-7 with stiff springs and big tires is far less likely to transfer/pitch weight forward and lighten the rear of the vehicle (i.e., cause rear wheel lockup) under hard braking.  If you follow their advice and leave the proportioning valve out, test the brake system cautiously and thoroughly in a safe environment...

 

Hydraulic pressure boost, if you mean the OEM type that receives pressurized fluid from the power steering pump, is well tested.  My Dodge Ram/Cummins 3500 truck and other diesel trucks (with no engine vacuum source) use a hydraulic brake booster.  GM has used this type of boost since the '80s on light truck brake systems.  I worked as a GMC truck dealership powertrain, chassis and warranty mechanic when these boosters first came into the market on 6.2L diesel powered full-size Jimmy and pickup models.  The booster can be reliable and provide the kind of pressure desired when the engine is running.  (Vacuum boost requires a running engine, too.  Under the best conditions, you will only get a few pedal pumps of vacuum boost once the engine stalls.) 

 

A major advantage with a hydraulic booster is the space saving over a large vacuum booster.  This can help with engine access, space for accessories and underhood tidiness.

 

Will you be using an iron tandem master cylinder with dual reservoirs?  Curious what master cylinder is paired with the hydraulic booster...Do you have a photo of the unit or link to the product website?  Does this setup require a special power steering pump supplied with the package, or do you use an OEM GM power steering pump for an LS engine?

 

Keep us posted!

 

Moses

[spdljohn]

The hydroboost kit is by Bosch.  I am purchasing it from http://powerbrakeservice.net.  To answer your question Moses, yes I will be using a tandem master cylinder 1 1/8" bore paired to the booster.  The power steering pump that is coming with the rebuilt LS engine is a new power steering pump, GM style that comes on the 2500HD pickups and Bob at power brake service says it is plenty good enough.  I am not using a hydraulic ram with the steering so the pump will be running just the hydroboost and the agr steering box I have on order.

 

I got the frame stripped down today, got the axles removed and power washed it.  Painted areas of concern with fluorescent orange paint.  My welder is going to sandblast it and take care of a few little things.  He has to rebuild a couple of the frame to body brackets for me.  I am then going to paint the frame with POR15.

 

This week should be a big week.  Santa claus (UPS) should be bringing some supplies to start getting her put back together.

 

I am going with an old man emu 3-3.5 inch suspension.  Looks like with that kit the front shackle support has to be moved forward about an inch.  Otherwise looks like that kit is plug and play.  

 

Ill be posting pics along the way.  Please feel free to add any advice.  It would be much appreciated.

 

The body is in somewhat bad shape.  Thinking of going with new fiberglass body.  Any thoughts on this?

 

David

[Moses Ludel]

Sounds like good progress around the brake system, David...As for the front shackle supports, since you have to move the brackets forward, this would be a good time to consider a front shackle reversal kit that puts the spring anchors at the front ends of the front springs.  The front axle then "trails" behind the anchors. 

 

The vehicle tracks better with the front axle trailing from the forward-mounted anchors—instead of the front axle being pushed down the road by OE rear anchors and the leaf springs.  When you encounter an obstacle off-road with the shackle reversal, the front axle will lift up readily instead of driving all the thrust into the frame at the OEM rear anchor positions before compressing (lengthwise) the springs and ultimately "popping" the front axle upward.  Since you're configuring the shackle position anyway, it is also practical to install reverse shackle brackets at this time and re-hang the shackles at the rear of the front springs. 

 

If you look at your OEM frame shackle brackets, they are a poor design.  AMC/Jeep punched a relief hole for clearing a frame rivet.  This relief hole also creates a weak area in the shackle bracket, and these factory bolt-on brackets are notorious for breaking or bending under load or during spring and axle articulation...another good reason for installing an aftermarket shackle reversal kit.  If you install a shackle reversal kit at the front end, take careful measurements to avoid altering the front axle location, wheelbase length or creating a front driveline length issue.  The front driveshaft must be able to extend and compress properly under full spring travel and articulation, without compromising the spline coupler engagement or bottoming the coupler under driveline compression.

 

Historically, I've not been a big fan of fiberglass bodies unless one lives in the Rust Belt.  Quality steel replacement bodies are available, they generally fit better, require less hardware adaptation and provide sufficient electrical grounds for body mounted D.C. devices and lighting.  I encourage those with recent experience around Jeep fiberglass bodies to add their comments.

 

Moses

[spdljohn]

Thanks Moses,

 

I've heard conflicting reviews of the shackle reversal kit.  I bought the old man emu suspension from rocky road suspension (rocky-road.com).  They advised against shackle reversal with the idea being that jeep designed it the way they did for a reason.  They claim that under hard braking conditions with the shackle reversal there was instability that could lead to loss of control.

Shackle reversal sounds great and makes sense intuitively. 

 

Question is why didn't jeep design it with shackle reversal to begin with?

 

If I did decide to go with a  shackle reversal kit it looks like the factory rear spring anchors are riveted into the frame.  Is this true and if so that must mean there are no nuts welded onto the inside of the frame to bolt into.  Am I mistaken?

 

Considering this jeep will be used for on road driving only it sounds like shackle reversal may be the way to go.

 

David

[Moses Ludel]

To separate the topics, we've moved the shackle reversal kit discussion to a new topic at:  http://forums.4wdmechanix.com/topic/486-front-shackle-reversal-on-a-1976-86-jeep-cj-4x4/.  For those interested in that discussion, please join us at the "Front Shackle Reversal on a 1976-86 Jeep CJ 4x4".

 

This brake topic will continue here...Please follow and contribute at these two topic discussions!

 

Moses

[60Bubba]

Looks like another Jamaica Beige CJ-7 getting some love.  Nice. It's just like my project (except you know what you're doing and have an awesome shop to work in.)