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WJ Grand Cherokee brake booster/mc swap

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First post, shoulda joined long ago. I've had my MJ since 'o5, it’s a charcoal silver shortbed Eliminator 2wd with TJ Rubicon wheels.

My truck was a zombie when I dragged it home. I gave it a new life, but it still liked to eat cars(brains to a zombie Jeep), since I've had two accidents where the other driver pulled out in front of me without adequate space for me to slow down. "Oh, I thought you could stop." Impatient college kids(both chicks), they need to take a physics class.


I've pieced my MJ back together and used it as a daily driver, and replaced the booster/mc with stock parts(back in '06), but the brakes still sucked. I lived with it for way too long.


I recently bought a newer bigger safer truck(wife’s request), and decided to try to sell the MJ. I started to wrap up the unfinished projects on it, but had no luck with the sale (y’all might have seen it on c-list and the big auction site). After this swap, I think it’s a keeper, again.


It all started when I decided to replace the rear flex hose, since it looked bad. Then I couldn’t get the brakes to bleed correctly, and came across the info for how to bleed the system for the rear load sensing proportioning valve here on this site. Thanks! But, the system still wouldn’t stay bled. Time for a new master cylinder.


I started looking into what other booster/mc combos would work as an upgrade. I came across the mid '90's XJ dual diaphragm swap, and one involving the '99-'04 WJ setup. I looked for both setups, and came across far more '99-04 donor possibilities in the parts yards. I picked up a good, useable WJ booster/mc for $80.


The swaps I read about involved the use of a ¼” spacer between the firewall and booster. This lowers the pedal by an inch, since most power brake systems use a roughly 4:1 pedal ratio. I didn’t like the way this setup looked or felt, so , after spending a good part of the day fabricating a spacer, I decided to chuck it and figure out a way to mount the booster directly to the firewall.


I had to grind out the large center hole to make room for the larger diameter pushrod housing on the WJ booster. There are also two vertical ribs on the firewall on each side of the booster mounting holes that I had to cut off about 3/8”, so the booster could be bolted to the firewall. I also clearanced the horizontal pinch seam above the booster by folding the downward-pointing lip inward, then I used a bottle jack and some scrap lumber to push the seam upward. Here are the left and right seams that had to be ground down:


After modifying the WJ pushrod by drilling the bolt hole out to 5/8” and grinding a flat for the stoplight switch, and modifying the WJ master-to-distribution valve hard lines with the old MJ line fittings flared in place, I bled the system, chased the stoplight Gremlins with a new switch, and finally got to go for one of the most satisfying test drives I’ve ever taken.


Of all the swaps and customizations I’ve done to my MJ over the years, this is by far the best one!


Anyone with a die grinder or Dremel can do the firewall modifications and not have to use a spacer. Modifying the WJ MC hard lines took a lot of practice to get familiar with the flaring tool.


If you have the basic mechanical knowledge necessary to replace a brake booster/master cylinder, I can’t say enough about how much of an improvement this swap will make to how your MJ/pre-’95 XJ will stop.


I did not take the time to snap any pics during the swap, but I will take some of the finished product soon. It looks like it came from the factory with a dual diaphragm booster.

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I'll have to take some better pictures after my camera charges up. Found the proper place I shoulda posted this, found a more detailed/picture-strong writeup here in the DIY projects.


Every forum is different, I hadn't been all over this one, and did some weak searches on the subject I posted here. Delete? Move?

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