YJ Master Cylinder Upgrade
YJ Master Cylinder Upgrade Short Cuts
by: Ron Hollatz

front view of installed master cylinder. Photo by Author
Side view of installed master cylinder. Photo by Author
notice the larger reservoirs in the new (right) master cylinder. Photo by Author
Sometimes you really need to count on your brakes. Bikini Trail at 1999 Dakota Territory Challenge. Photo by Laurie Hollatz

When I had the Dana 44 built for the rear of my 94 YJ I had the shop add rear disc brakes instead of the stock drums. The setup uses Super Bell cast caliper brackets with 11" Lincoln Versailles rotors and small GM calipers with emergency brake. I've had 4 wheel discs on other vehicles and was always impressed with the stopping power they provided but I was surprised by the results on my Wrangler, there was little if any braking improvement. By adding heavier axles with larger tires the stock braking system seemed to be overwhelmed. This really became a problem for me at Moab when I needed to add regular braking to the compression braking while inching down steep obstacles. I get a lot of compression braking from my Atlas transfer case and 4.56 gears but there are times when I wanted to gently set a tire down on a rock and couldn't do it. I was also having some problems with my Atlas popping out of low range (since fixed) on steep descents and I would have to slam on the brakes. Nothing is worse than feeling the pedal go to the floor and the Jeep is still accelerating down hill.

From the number of posts I've seen on the web it seems there are a lot of other people out there with this same problem. Over the last couple of years it's become kind of a quest for me to find a way to improve my braking without spending a small fortune. The consensus seems to be the master cylinder on the YJs does not push enough brake fluid for the rear calipers. The master cylinder's reservoir for the rear drums is much smaller than the one for the front discs. If you look at a master cylinder for a vehicle with 4 wheel discs the reservoirs are about the same size. One solution that was recommended is to go to an adjustable proportioning valve to adjust the front and rear pressure. I didn't try this because I felt I needed more stopping power all around. With a CJ the solution is to go to a GM master cylinder from a vehicle with 4 wheel discs (usually a 79 Pontiac Firebird) but when Jeep started building Wranglers they went to a different bolt pattern.

First off there are solutions out there if you are willing to spend the money but I was looking for an off the shelf solution I could pick up at any parts store. I already have to carry a lot of custom spare parts on trips because I don't want to miss out during an event while I wait for parts to be shipped. I spoke with most of the brake kit companies and their response was either you don't need anything better or they would sell me the correct master cylinder at a premium price. Even if I purchased the master cylinder from them I wouldn't know the application it came from if I needed to replace it in the future. Recently there has been a lot of talk about adapting larger power boosters or even a hydroboost system from a larger vehicle but these were expensive swaps requiring some major modifications and I would end up having to carry expensive spare parts. I was to the point of spending the money (~$500.00) on the hydroboost system and I went to a local off-road shop, 4x4 Unlimited, to see if they could do any better on the price.

A Solution

4x4 Unlimited is located in Elgin, MN and while most of the off-roading in the Midwest features full size vehicles and lots of mud they have a loyal following of rock crawlers. I spoke with Tom Steiger about what I was looking for and he had a few suggestions. The first thing he wanted to do was take a look at how the rear calipers were setup and to make sure they were bled properly. A common problem with rear calipers is the bleeder screws may not be located properly. This leads to air being trapped in the caliper even after multiple bleedings. Mine were fine but if they weren't the solution is to remove the calipers from the brackets and use a piece of wood in place of the rotor. This allows you to move the calipers around and make sure the bleeder screw is at the highest point. The other thing Tom wanted to check was the brake cable adjustments. The GM calipers are real touchy on their emergency brake adjustments and you want to make sure the cable allows the emergency brake to fully engage. The emergency brake also has to be used regularly to keep it in adjustment. The last thing Tom looked for was if mud was packed into the caliper. This is pretty common and it is best to clean out the calipers after any trips through the mud. Everything seemed to be working correctly so Tom and I discussed the options available.

The shop does quite a few hydroboost conversions and while they work real well on full size vehicles or highly modified trail Jeeps. Tom does not feel it is the best solution for a daily driven Jeep. The hydroboost system gives you a lot of braking power and for normal day-to-day driving you have to be real careful you don't put yourself through the windshield. The hydroboost system is also dependent on pressure from the power steering pump so if you stall the vehicle while crawling over rocks you will still have manual brakes but after a pump or two you will lose the hydroboost. Tom felt the best solution would be a master cylinder with a larger bore and more fluid capacity. He recommended the Firebird master cylinder but when I told him about the change in bolt pattern he started measuring. Its turns out the master cylinder in his Suzuki Samurai (I don't know if this is the stock pattern since not much of the Samurai is stock) is the same bolt pattern. He had replaced his with a Ford master cylinder so we started checking applications. Tom decided the master cylinder from a 78 Mercury Marquis with hydroboost and rear disc brakes would be the proper donor. Thinking we would have to make a new rod for the brake pedal we pulled out the old master cylinder. We were surprised to find the Wrangler came with an adjustable rod so fabrication was not necessary. When we put the new master cylinder in place (after a thorough bench bleeding) we found a bit of interference between the booster and the larger master cylinder but by adding a washer to each side as a spacer it bolted right on. The other advantage of the washers is the rod ended up needing no adjustment. We bled the lines one last time and it was ready to go.


My initial reaction was it felt like a brand new set of brakes. When I mashed on the pedal it didn't go all the way to the floor. I was also happy the brake pedal was more sensitive, not as much pressure was required to slow down. With the old master cylinder it seemed like I was always stomping on the pedal but with the new one I could feather the pedal a lot more. Driving around town was a lot less stressful since I could stop normally and didn't need to plan braking ahead of time. While the new master cylinder has made a big difference I don't want to mislead anyone. I can't lock up the 33" Swampers at 55 mph and it still doesn't stop like a car. It does however feel a lot more comfortable to drive and since I only spent about $30.00 on the master cylinder if I decide to change it in the future it won't be any big deal. At the Dakota Territory Challenge this year I had none of the problems I had experienced at Moab. The brakes worked as well as I expected and I never had that sinking feeling in my stomach as I tried to slow down before I ran off a cliff. I really feel it was a worthwhile upgrade and I feel no reason to spend the extra money on one of the custom systems. One of the real nice things about it is most auto parts stores can get this master cylinder in typically one day, making it easy to repair if I'm away from home.


I've been using this master cylinder for a while now so I thought it was time for some updates. One thing I failed to mention on the install is you will need adapters for the YJ lines to fit into the new master cylinder. The easiest way to do this is take the short lines from the proportioning valve to the master cylinder along with you when you purchase the new master cylinder. Most parts houses will carry the adapters you need.

Chris Waterman sent me the part numbers for the master cylinder he has been using. The rebuilt master cylinder from Raybestos is part number MC36468. He also mentioned that the same master cylinder can be found on Lincoln and Ford land yachts (full size cars) from roughly '74 through '78.

Tom Steiger and I recently spent some time going through my brake system after a front and rear axle swap. We found that the adjustable rod from the brake booster to the master cylinder need to be lengthened 3/8". This really helped the feel of the brake pedal. At the same time we removed the stock proportioning valve and added a Wilwood adjustable proportioning valve to the rear brakes. I haven't driven my Jeep much since these changes, but I'll keep you informed.

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