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Dual Diaphragm Brake Booster Upgrade for a CJ-7 Jeep


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My new-to-me CJ-7 has been getting worked over for all the legal/safety related parts as it will be my daily driver for the next year or so. I've done LED tailights, wipers, front markerlight/turn signals, windshield washer pump and tubing, horn, etc. The last thing that was bothering me was the braking. It stopped, but it required a fairly heavy foot. Pads, drums and rotors seemed healthy, so I assumed it was either a simple matter of big heavy wheels and 33" tires with a 31 year old system, or an issue of faulty components under the hood.

Upon investigation, the booster seemed to be pretty rough. The boots were nothing but black goo, and I didn't seem to have any residual vacuum after shutting off the enigne. For that matter, it didn't seem to have much with the engine running.

I decided to replace my single diaphragm setup with a dual. I sourced one from Restoration Performance. It is a dual diaphragm booster with a traditional cast iron master cylinder made by MBM. The kit comes with an adjustable pedal linkage and a new mount bracket. Install was relatively straight forward, though getting to the parts under the dash required the usual contortions.

I bench bled the master before installing, and used a combination of a vacuum bleeder and some pedal pressure bleeding. I think I actually bled far more than necessary, but after seeing the nastiness that came out for the first few ounces, I wanted to ensure none of the old fluid was left. This was probably also beneficial since I switched to a synthetic fluid.

After a few tentative stops from neighborhood speeds, I double checked for leaks and took it out on a quiet country road and did some progressively more aggressive stops. The pedal has great feel, and normal stops now require nothing more than steady toe pressure, more like a current generation car or truck. With a little more pedal effort, I could easily sling stuff off the seats under braking. I didn't try to lock them up, as my experience is that can get pretty hairy with such a short wheelbase, but I was able to stop hard enough to hear the tire tread squirming against the asphalt.

Overall, I think this was about the best $185 restoration/upgrade money I've spent. The fit and finish of the parts were good to excellent, and the increased performance is pretty much staggering. If I had unlimited time and money, I'd like to have been able to replace the booster and master cylinder with stock parts and then use the upgraded parts to see how much difference there was between the two. I'm now pretty convinced I wasn't getting much if any boost from the old system. I should hook up my vacuum bleeder to the old booster to check...At any rate, I'm now more confident that I can safely stop under any normal circumstances.

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This is very helpful information for other CJ owners, 60Bubba!  What a dramatic difference, and clearly from just the booster and master cylinder change.  Iron is still my favorite master cylinder material.

 

The MBM master cylinder and Restoration Performance dual diaphragm booster sound like the way to go.  The OEM boosters and check valves are now long in the tooth and likely in need of replacement.  Good to change out the master cylinder at the same time, as the added boost would overwhelm worn master cylinder seals.  Cost, overall, seems reasonable.

 

As for the switch to synthetic brake fluid, unless stated that it will mix readily with conventional DOT 3 or DOT 4, you did need to remove and flush out all of the original fluid.  DOT 5, specifically silicone brake fluid, is not compatible with DOT 3 or DOT 4.  DOT 5 must be replenished or topped off with DOT 5 as well.  Never mix this chemistry!

 

I like a vacuum bleed, as it pulls all old fluid and debris from the wheel cylinders and caliper chambers.  Pressure bleeding will force debris to the cup seal edges of the wheel cylinders or the caliper piston gaps and leave it there.  For a complete fluid change, I use a pneumatic vacuum bleeder that has a large dirty fluid reservoir, bleeding from the caliper and wheel cylinder bleeder valves.

 

Please share some photos of the new master cylinder and booster in place!  Use the "More Reply Options" below the editor box (right side) to find and place your photos.

 

Moses

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Here are a few pictures after the upgrade.  As Murphy would have it, I didn't get any during the install.  Too messy to handle the iPhone in those conditions!

 

Feel free to respond if anyone has any specific questions.

 

 

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Wow, 60Bubba, this is a substantial upgrade!  The master cylinder looks larger capacity, the booster dwarfs the OEM. 

 

As a footnote, there should be a slight amount of pushrod play at the master cylinder with the pedal completely retracted.  If it was factory set, it should include this slight play.  The play allows the master cylinder piston(s) to retract completely and not block the compensating port.  If the pushrod does not retract completely, it can trap brake fluid in the system and cause brake drag or even wheel lockup.

 

This is a general concern around setting up a master cylinder pushrod.  Power booster pushrods into the master cylinder are set with a gauge and specific height, presumably yours was gauged if sold as a master cylinder and booster assembly.  (Manual brakes simply rely on the pedal free-play, which your system requires, too.  Pedal free play on your CJ should be 1/16" to 1/4".  This is movement felt before the pedal has any kind of resistance from the master cylinder.)  The play I'm describing is simply to assure full piston retraction when you release the brake pedal.  Worth a peek.

 

Moses

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I did read something in another booster upgrade thread somewhere about the need for some freeplay.  It didn't have the thorough description you gave, but it was enough to make me pay attention to it.  I did have to adjust the pushrod length to make sure there was no pressure being applied when the brakes weren't applied.

 

I did notice one related issue: my brake pedal doesn't seem to have a return spring.  Is that stock, or am I missing a part?  Right now, the pedal has about 1/8" of freeplay, but the weight of the pedal does essentially rest against the booster piston when the brakes aren't applied.  It doesn't seem to be creating any drag, but I was curious if I needed to install a helper spring to keep the pedal in the full "up" position.

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60Bubba (Case)...Yes, there is a return spring (Mopar Part #J5351050, likely available in the Jeep aftermarket as well).  Here is the pedal assembly and all related parts, the spring is item #11:

 

1981-86 CJ-7 Brake Pedal Assembly.pdf

 

This will help clarify the location of parts, too.  Very pleased that you have a safe, quality brake system now!

 

Moses

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  • 2 years later...

Looking for the part number for this kit, also would it work on an 86 CJ7 with an AMC 304 V8? Never done brake work so I might have a few questions during install also. New to the Jeep community and hoping to do most of my own work.

 

Thanks!

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Thanks for this post! I'm also planning to upgrade my MC and single-d booster on my 1983 CJ8. I did put a vac gauge on the booster vac line fitting with engine running and saw around 18psi at idle. That's with a rebuilt 258 with 4.0L head and MPI conversion. When I do the pedal test, it's clear that the booster provides assistance when compared to no boost (test with engine not running). However, I'll be interested to see the difference with the dual-d booster...

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