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Ford 8.8" Rear Disc Brake Conversion on 1986 Jeep CJ7 Dana 44

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I've got a 1986 CJ7 258 Weber carb, factory Dana 44 rear (re-geared to 3:73's with Lockright locker) narrowed down Scout Dana 44 front with 3:73 gears, Spartan locker, old-style 6 bolt wheel hubs, stock factory front discs, Jeep T18 4 speed to a Scout/Dana 30 transfer case, power steering cooler, AMC Concord A/C now, onboard air with a tank, 35" BFG muds on 10" rims.


I recently added Ford 8.8 rear discs to the Dana 44 rear.  I had to have the bolt pattern redrilled to 5 on 5 1/2 and cut out the middle hole bigger for the flange. I had to elongate the mounting holes and bracket holes, mounted everything up and discovered the rotors set out 1/4" too far on the hat of the rotor. I added a 1/4" spacer, and everything bolted up.


I hooked up the calipers and bled the lines. Had front brakes but little to no rear brakes. Went to the local u pick it junkyard and got an '05 TJ Rubicon proportioning valve. I checked all the lines and installed the proportioning valve. It seemed worse!


Questions: 1) Should the proportioning valve be up near the master like the Rubicon, or is it okay to have it in the CJ7 location on the frame? 2) Should I still get an adjustable pro valve and keep the Rubicon proportioning valve, or 3) should I go back to the CJ7 proportioning valve and add the adjustable valve near the master?  Or should I upgrade the master?

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Very well-equipped Jeep CJ-7 4x4, "LastCJ7"!  Interesting brake system, trust all safety concerns were addressed with the alignment and fit of the rotors and calipers.  There is huge rotational force with braking, and the wheel studs, rotors and caliper mounting brackets must each follow OEM engineering principles and material standards.  Presumably, you did your homework here...Is this a proven and safe conversion? 
Sounds like the issue with the proportioning valve could be proper bleeding technique for the '05 Rubicon type valve.  We'll address that, as you likely do not have fluid pressure at the rear brakes from what you describe.  Before bleeding properly, though, I'll comment on your other two questions.
Regarding the mounting point for the proportioning valve, either position would work.  Since pressure from the master cylinder to the valve remains uniform, the location should not be an issue.  When the master cylinder bore is not “stepped”, the line pressure would actually be equal on each sub-system (front and rear brakes), at least from the master cylinder to the proportioning valve.  Sometimes the master cylinder bore is stepped in size, in which case the line pressure leaving the master cylinder differs per line.
Many manufacturers mount the proportioning valve at the frame, directly below the master cylinder.  The idea is to make it accessible, especially for bleeding or centralizing the safety switchover valve (a time-honored, challenging feature on later Jeep CJ braking systems).  You likely have encountered the combination valve bleeding issue with your stock brake system.  On your original CJ-7 combination valve, a special tool holds the switchover valve in position during the bleeding procedure.
Your '05 Rubicon proportioning valve is designed for a four-wheel disc brake system.  It was also intended for the Rubicon master cylinder, which delivers the correct volume of brake fluid to the front and rear.  In considering master cylinders, it's worth noting that disc brake caliper pistons typically require more fluid volume to actuate than drum brake wheel cylinders.  Note that the master cylinder reservoir for disc front brakes/drum rear has a larger front brake reservoir, reflecting the need for more fluid displacement during the braking process.
It's unclear whether your rear disc brakes are getting enough fluid volume, and that's something you need to determine.  If there is an issue with the master cylinder's per stroke fluid displacement and not enough brake fluid to the rear brakes, the rear calipers will not apply properly.  You would need to retrofit something like the Rubicon master cylinder to remedy that problem.  
If you determine that the rear brakes do not get enough brake fluid volume with your CJ-7 master cylinder and decide to use a Rubicon master cylinder, be sure you mount the retrofit cylinder properly.  The master cylinder piston must fully retract with the brake pedal released!  If the pedal height is wrong or the power booster pushrod is too long, the brake master cylinder piston will stay in a partially applied position.  If the master cylinder piston is either blocking or past the compensating port(s), this can trap fluid in the brake system and cause brake drag or even wheel lockup.
If bleeding the system does not resolve the rear brake apply issue, and you're certain that fluid under pressure is reaching the rear brakes, then you do have a fluid displacement problem: too little fluid volume per pedal stroke to actuate the rear brake calipers.
What you're calling the proportioning valve is actually a "combination valve".  Here are 2005 Jeep TJ Wrangler factory designated combination valve functions:
     "The combination valve contains a pressure differential valve and switch and a rear brake proportioning valve. The valve is not repairable and must be replaced as an assembly if diagnosis indicates this is necessary...The pressure differential switch is connected to the brake warning light. The switch is actuated by movement of the switch valve. The switch monitors fluid pressure in the separate front/rear brake hydraulic circuits...A decrease or loss of fluid pressure in either hydraulic circuit will cause the switch valve to shuttle to the low pressure side. Movement of the valve pushes the switch plunger upward. This action closes the switch internal contacts completing the electrical circuit to the red warning light. The switch valve will remain in an actuated position until repairs to the brake system are made...The proportioning valve is used to balance front/rear brake action at high decelerations. The valve allows normal fluid flow during moderate braking. The valve only controls fluid flow during high decelerations brake stops."
Here's how to test the combination valve, according to the factory procedure.  You need to have the brake warning light switch hooked up or at least check the switch's continuity in the different modes, using an ohmmeter...You may be able to use the OEM CJ-7 brake warning light for this function.  That would preserve the light's function:
Pressure Differential Switch—(1) Have helper sit in driver's seat to apply brake pedal and observe red brake warning light. (2) Raise vehicle on hoist. (3) Connect bleed hose to a rear wheel cylinder and immerse hose end in container partially filled with brake fluid. (4) Have helper press and hold brake pedal to floor and observe warning light.
1) If warning light illuminates, switch is operating correctly.
2) If light fails to illuminate, check circuit fuse, bulb, and wiring. The parking brake switch can be used to aid in identifying whether or not the brake light bulb and fuse is functional. Repair or replace parts as necessary and test differential pressure switch operation again.  Note: If warning light still does not illuminate, the switch is faulty. Replace combination valve assembly, bleed brake system and verify proper switch and valve operation.


Now you can bleed the combination valve and brakes.  Start with your current brake system (Rubicon combination valve installed with OEM CJ-7 master cylinder).  Make sure the combination valve aligns in the stock '05 Rubicon way, with the front and rear lines attached in their correct positions.  Here's the factory procedure for manual bleeding and pressure bleeding, beginning with manual bleeding:

Use Mopar brake fluid, or an equivalent quality fluid meeting SAE J1703-F and DOT 3 standards only. Use fresh, clean fluid from a sealed container at all times. (1) Remove reservoir filler caps and fill reservoir. (2) If calipers or wheel cylinders were overhauled, open all caliper and wheel cylinder bleed screws. Then close each bleed screw as fluid starts to drip from it. Top off master cylinder reservoir once more before proceeding. (3) Attach one end of bleed hose to bleed screw and insert opposite end in glass container partially filled with [clean and fresh] brake fluid. Be sure end of bleed hose is immersed in [clean] fluid. (4) Open up bleeder, then have a helper press down the brake pedal. Once the pedal is down close the bleeder. Repeat bleeding until fluid stream is clear and free of bubbles. Then move to the next wheel.

Pressure bleeding procedure:


Use Mopar brake fluid or an equivalent quality fluid meeting SAE J1703-F and DOT 3 standards only. Use fresh, clean fluid from a sealed container at all times.  Follow the manufacturer's instructions carefully when using pressure equipment. Do not exceed the tank manufacturer's pressure recommendations. Generally, a tank pressure of 15-20 psi is sufficient for bleeding. Fill the bleeder tank with recommended fluid and purge air from the tank lines before bleeding. Do not pressure bleed without a proper master cylinder adapter. The wrong adapter can lead to leakage, or drawing air back into the system. Use adapter provided with the equipment or Adapter 6921.
As for bleeding, I suggest that you vacuum bleed the system if the combination valve will tolerate that kind of bleeding while remaining in a centered mode.  I've used a compressed air actuated vacuum brake bleeder for years and believe this is the safest way to bleed brakes: fast, thorough and intended to completely evacuate debris and old fluid. Most often, one person can easily bleed the system.
Envision pressure or manual bleeding:  Either of these methods allows debris to remain near the wheel cylinder sealing cups or caliper piston edges while fluid under pressure exits from the centrally located bleeder valve.  By contrast, a vacuum bleeder at a wheel cylinder or caliper does exactly the opposite: It draws the debris and old fluid out of each wheel cylinder or caliper! 


Note: I use a vacuum bleeder to complete purge my brake systems of old fluid—periodically.  This extends rubber seal life, reduces risk of corrosion damage and improves braking performance...If there's interest, I'll start a new topic post on this subject...
You now have information on the combination valve's functions and how to troubleshoot the valve.  Bleed the system properly, and see if that provides normal apply pressure at each wheel's brakes...If the rear brakes still fall short and do not have adequate pressure at the calipers, you need a four-wheel disc master cylinder.


There are pinpoint diagnostic tools for brake pressure troubleshooting.  A four-wheel brake pressure testing kit can read actual brake apply pressure at each wheel.  Dealerships, brake shops and high performance automotive shops can often perform these tests.  This tool can pinpoint the hydraulic system trouble we're addressing here...I have a brake pressure testing kit and will share details at the "Diagnostic and Specialty Tools & Equipment" forum.
If you have further questions, post a reply!

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Hi, Moses, great info as always.  What I did not tell you is that the bleeder screws on the Ford 8.8 calipers are frozen, so I bleed them from the lines.  I was told today that I can not do that...You must bleed from the bleeder valves to get all the air out. So I will be using an easy-out in a day or two, or worse case install new calipers. Then I will see if that helps. Thanks

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Keep us posted, and keep my comments in mind if the results are not what you expect...Vacuum bleeding would be a good idea with the easy-out metal debris.


Here for more questions!

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I got one bleeder out and had to buy a new caliper. I installed them and pumped up the pedal,
bled back and forth and added fluid. I got some air out, seems better.  Will get a buddy over to help me do the two man bleed.  This is a start!

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Curious to see how this works out with the full brake bleed.  Comes down to whether you'll get enough stroke/fluid volume at the rear brake circuit while using a CJ-7 master cylinder.  You need enough fluid per pedal stroke and adequate apply pressure at both the front and rear calipers.  If this is not the case, consider the TJ Wrangler Rubicon four-wheel disc brake master cylinder!


On another note, if you're using your disc/drum CJ-7 master cylinder, make sure there is no residual valve on the rear brake hydraulic circuit.  Otherwise, the rear brake pads will drag on the rotors with the pedal released, causing quick pad wear, brake fade or even wheel lockup.  This step is often overlooked on rear disc conversions.  I've opened a new "topic" on this important safety subject.  See this discussion begin at http://www.4wdmechanix.com/forums/index.php/topic/110-rear-disc-brake-conversions-master-cylinder-needs-and-the-drum-brake-residual-pressure-valves/.


I'm taking photos of the brake pressure test equipment and will post at the tools forum...4x4 owners who work on their vehicles and professional techs will appreciate this diagnostic tool.


Have a safe, fun 4th of July holiday.  Looking forward to the update on your Jeep CJ-7 four-wheel disc brakes.



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Well, I'm not sure what your original motive was for the disc rear brake conversion, but the findings are not that surprising.  Brake bias is to the front, and your Jeep CJ-7 has adequate factory disc brakes there.  The drum rear brakes on a later CJ-7 were also sufficient for most uses.  Are you satisfied that the brake apply pressure is sufficient at the rear brakes?


Let's put some rays of sunshine into your effort around this 8.8" rear disc brake conversion.  Since you went ahead and did it, the positive gains should be considered.  For openers, disc brakes have several trail use advantages over drum brakes:


1) Resistance to fade in hard rock crawling and continual application of the brakes; this also applies to trailering.


2) Ability to immediately work well after crossing a stream, with less risk of damaged parts from water crossings.


3) Winter benefits over drum brakes (RareCJ8 laments his drum rear brakes); less risk of frozen parts or ice lock-up when parked.


4) Ease of service around pad changes; calipers are accessible. 


5) Easier visual inspection for wear or defects; troubleshooting is quick and simple.


6) You have a contemporary brake system (8.8" Ford) with easy parts availability on a weekend—or in the middle of Podunk. 


What was the source/application for these rear disc brakes?  I'd like to compare the OEM bore diameters between your CJ-7 master cylinder and the Ford application's master cylinder.  To be completely fair about the effectiveness of the "new" disc rear brakes, let's determine whether the CJ-7 master cylinder is applying the same pressure to these rear calipers as the Ford OEM master cylinder.


Again, your findings are not unusual.  Rear braking on a late CJ-7 can be readily met with OEM drum brakes.  Now, if you had the '55 Jeep CJ-5 featured in my Jeep CJ Rebuilder's Manual 1946-71 Edition, that would be another story.  9" x 1-3/4" brakes are inadequate, front or rear, by any standard!  I've often noted, however, that 11" x 2" drum brake conversions work very well on the early Jeep CJ and military models.  While disc brakes have many advantages, effective rear braking can be met, most of the time, with sufficiently sized drums and brake shoes.



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1996 Ford 8.8 rear disc brakes on a 1986 Jeep CJ7 factory Dana 44 rear axle.  2 Ford rear passenger side hoses, 1/4" spacer, re-drilled to 5 on 5-1/2" and hole cut out to fit axle shaft.  Modified rear shoes.

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I see the dramatic difference in the original wheel bolt circle and your drilling new wheel stud holes for the Jeep 5 x 5.5" wheel bolt diameters.  Also see the "lathe cut-to-fit" of the rotor centers to fit your CJ-7 axle shaft hub. 


The 8.8" axle must be from a Crown Victoria RWD car?  Or a Mustang?  Curious what donor application, as I'm still trying to compare the master cylinder pressure differences between your CJ-7 Jeep master cylinder and the donor car.  What was the 1996 Ford donor model?



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Ah, why didn't I consider the Explorer?  Smaller wheel bolt circle, four-wheel disc brakes, 8.8" rear axle!


Comparing master cylinders on the basic TJ Wrangler (disc front/drum rear brakes) versus a TJ Wrangler Rubicon (four-wheel disc brakes), I picked 2005 model year as an example.  Both brake systems use the same master cylinder that year:


1 04798157 1 MASTER CYLINDER, Brake

R4798157 Mopar Remanufactured Part


When you get to the proportioning valve, there are two part numbers for the 2005 Wranglers.  These two numbers apply to all models of the Wrangler, both disc front/drum rear and four-wheel disc brakes.  The proportioning is the same part number for each of these late systems.  (I describe the function of the proportioning valve in an earlier post at this topic, and you can see why the same valve works for both systems.) 


The Mopar part number distinction for the proportioning valve is not whether the vehicle has disc front/drum rear or four-wheel disc brakes.  One valve is for ABS models, the other valve is for non-ABS models:


VALVE, Proportioning
05083807AA  [bGK]  (anti-lock brakes)
05083808AA  [bGA,BRW]  (without ABS)


As for master cylinder bores, your stock 1986 CJ-7 master cylinder for power brakes can be either 1" diameter or 15/16" diameter, depending upon the cylinder manufacturer and vendor.  The Ford Explorer OEM master cylinder should have a 1-1/16" bore. 


The rear/drum brake reservoir and port is at the front of your CJ-7 master cylinder.  The bigger reservoir, closer to the firewall, is the disc front brake portion of the master cylinder.  Here is a photo with a nice description from one aftermarket master cylinder supplier:



This is a typical aftermarket replacement master cylinder for a 1986 Jeep CJ-7 4WD model with power booster.  The bore is 15/16", although some designs for this application are a 1" bore size.

(If you cannot open this photo, consider becoming a forum member for free—you'll have full access to all features and can join in the discussion, too!)


I researched bore sizing to determine how your Jeep CJ-7 brake master cylinder pressurizes the brake system then compared that with a 1996 Ford Explorer master cylinder—specifically to determine the braking system pressure that reaches the Ford Explorer rear disc brake calipers. 


The brake pedal leverage and power booster determine the mechanical pressure pushing against the master cylinder pistons.  What we want to know here is the fluid pressure that the master cylinder delivers to the rear disc brake calipers.  This is determined by the area of the master cylinder bore size


Let's assume that your CJ-7 Jeep cylinder is the smaller 15/16".  The formula for the area of the bore size would be Pi times the radius squared:


15/16" = 0.9375" bore diameter
15/32 = 0.46875" radius of bore
Pi = 3.14159265359


3.1416 (rounded up Pi)  x 0.46875 x 0.46875 = 0.69 square inch


For the Ford Explorer master cylinder area:


1-1/16" = 1.0625" bore diameter

17/32" = 0.53125" bore radius

Pi = 3.14159265359


3.1416 (rounded up Pi) x 0.53125 x 0.53125 = 0.8866 square inch


Expressed as pounds per square inch fluid pressure coming out of the master cylinder, let's apply 750 pounds of mechanical force, using the brake pedal leverage and power booster:


Jeep CJ-7 master cylinder fluid pressure into the system = 750 divided by .69 = 1087 PSI


Ford Explorer master cylinder fluid pressure into the system = 750 divided by .8866 = 846 PSI


With the same mechanical force of 750 pounds applied to the master cylinder pistons, the CJ-7 cylinder will pressurize the brake system at 1087 PSI, and the Ford Explorer master cylinder would pressurize the brake system at 846 PSI.  This is the fluid PSI pressure going to the front and rear brake calipers.  At that stage, the actual apply pressure of the caliper pistons against the rotor faces is determined by the bore size of the caliper pistons.


The actual pedal pressure/leverage point and brake booster in this case is stock CJ-7.  The difference in master cylinder bore sizes has the Ford Explorer rear disc brakes getting more apply pressure with your CJ-7 Jeep master cylinder than the same brakes would get on a Ford Explorer.  The end result is more brake pressure at the retrofit rear calipers in your CJ-7 than the brake pressure in a Ford Explorer—applying the same mechanical force (pedal and booster) at the master cylinder.


You should notice some difference with these rear discs over the OEM Jeep CJ-7 drum brakes.  From what you share, the good news is that you're apparently not over-powering the rear brakes, causing rear wheel lock up or running the risk of spinning the Jeep out on a slick surface.  Your CJ-7's beefy Scout II front axle, 35" tires, stiffer suspension, plus the shorter wheelbase and shorter overall length than a Ford Explorer, also contribute to the brake system's balance. 


The difference in master cylinder apply pressure is not dramatic, even less if your CJ-7 master cylinder is actually a 1" bore and not 15/16".  Your use of the late Jeep TJ Wrangler Rubicon four-wheel disc brake proportioning valve also helps keep brakes in balance front to rear.


By using the Jeep CJ-7 master cylinder with its slightly smaller bore, you've booster the brake pressure to the rear calipers when compared to a stock Ford Explorer.  This is all relative to the brake pedal and booster force.  The best test is the real world.  Overall, you do not want the rear wheels to lock up under hard braking or on a slick surface.  This makes the proportioning valve important.


If you had installed an aftermarket manual proportioning valve on the rear brake system, and adjusted the valve to prevent wheel lockup, the setting would likely be the same as how your brakes apply now.  The Jeep CJ-7 chassis dynamics are different than the Explorer, and the wheelbase is shorter.  You do not have symptoms of over-braking at the rear wheels...That's how the brakes and the proportioning valve should work!


As a footnote, you shared that you now have "new" rear calipers after the bleeder valve issue.  The photo at your earlier post shows an older caliper and rotor.  If these parts are used, the brake test results you get now may be different if you install new rotors, pads and calipers.  There could be an improvement in rear brake performance with all new parts.  If so, be cautious until you're certain that the rear brakes still will not lock up on slick surfaces or under hard braking.



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Glad you're happy...I am, too!  As for why disc brakes work well up hill or down, the reason is that they are not like drum brakes:  Drum brakes, at least modern, self-energizing types, feature a short shoe lining toward the front and a long brake lining toward the rear of the vehicle.  This design takes advantage of "self-energizing", drum rotational force to have the initial contact shoe press against the drum, stop at the anchor bolt and move outward from rotational force.  


Rolling forward, this self-energizing or rotational force is at the rear or long brake lining, which is forced against the drum.  With this same direction of rotation, the front or short lining brake shoe relies more upon hydraulic force from the wheel cylinder to push the forward lining against the brake drum.  Envision the long lining shoe wedged against the anchor at the top of the backing plate; with applied brakes, this shoe attempts to rotate with the drum and can only move outward; that mechanical force is significant and called "self-energizing".  It takes advantage of both hydraulic pressure and the physics of self-energizing force.


When you back the vehicle up with this short lead shoe and long rear shoe, the rotational force is the opposite: the short shoe exerts the extra energy of the rotational force (short shoe stops at the anchor bolt then pushes outward).  The long lining shoe (facing the rear of the vehicle) relies more on hydraulic force.


Where this is really evident is the emergency or parking brake on a self-energizing drum brake system with rear drum brakes that incorporate a mechanical emergency or parking brake function.  Ever notice that you can back the vehicle up with the rear drum E-brake on?  At least fairly easily.  When you try that moving forward, the vehicle immediately grabs or won't even move.  This, again, is the long shoe versus short shoe and self-energizing brake effect.  It works with either the hydraulic or mechanical (parking) brakes applied.  It also suggests not parking the vehicle uphill and relying totally on the drum brake parking brakes!


Note: Many rear disc brake systems now have a drum and shoe E-brake.  If the shoe lining length is equal on both shoes, the parking brakes should, theoretically, hold with equal force in either direction of rotation: uphill or down.


So, you're right!  With disc brakes at all four wheels, the brake apply pressure at each rotor remains the same, regardless of the vehicle's direction of movement.  There is, of course, the shift in vehicle weight bias when applying the brakes at speed or when the vehicle is aimed uphill or downhill.  At speed, weight and brake bias would go to the axle facing the direction of vehicle movement.  When on slopes, bias is at the axle that bears the most weight.  Weight bias changes with the degree of slope and direction your Jeep is pointed: uphill or downhill.


In addition to this, you have more effective brakes when backing up because the rear brake rotors and calipers apply more force than the original, self-energizing drum brakes were capable of doing when the drums rotate backwards!



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When I could only get one of the bleeders out, I got a new caliper from Auto Zone. I should have gotten two but I wanted to see how the brakes were gonna work, and I should have gotten new pads. I will
more than likely do both down the run. I'm wiping the rotors off with the old pads, LOL.


As for the E-brake cables I cut some of the spring on the CJ cables and just wrapped it over the lever for now with zip ties I'm going to Home Depot to get the cable clamps/saddles. What do they say, "Never saddle a dead horse." LOL


I Also keep forgetting to say that I use your Jeep CJ Rebuilder's manual for tips and insight. I will try to take more pictures soon!

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A work in progress...Glad you're dealing with the E-brakes, you do need a backup and parking aid. 


Thanks for the kudos on the Jeep CJ Rebuilder's Manual (Bentley Publishers), the 1972-86 edition for your Jeep CJ-7.  I enjoyed getting to know your 4x4 club's members at my Bentley Publishers workshop in Cambridge, MA a few years back!  Glad we've kept the communication going since...



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