Brake booster & master cylinder vendor quality

I have replaced the vacuum brake boosters once, and master cylinders on both my oldest XJ jeeps (85 and 87) twice, and the last time was not too long ago, and I am doing both (both jeeps, both parts, $$$) again tomorrow. In the past I did them one part at a time as they failed. I always use, and have used Autozone rebuilt parts (for like 20 years).

This time I am using new cast iron master cylinders, but I was not able to get new brake boosters (vacuum).

I think the rebuilt master cylinders leaked out the rear somehow and brake fluid got to the rubber diaphragms on the brake boosters causing early failures.

I have yet to replace or work on the proportioning valves, but I am wondering if they are causing part of my problems as the rear brakes never seem to kick in very well, but that may just be typical XJ drum brake adjuster problems. Not sure yet, but looking for advise and others experience if this rings any bells.

I never have found other external leaks, at least not recently, but I seem to loose fluid on the front side of the master cylinders reservoir (rear brakes) side early in the failure mode with no signs of where they are leaking, except once when it was bad out the rear side onto the brake booster.

This will be the third set of master cylinders on both jeeps in 4 to 6 years, and I am only getting about 15,000 to 20,000 miles out of the rebuilt master cylinders. That is why I bought new master cylinders this time. Note that the new master cylinders are cast iron, and the last set of rebuilts were aluminum.

Anyone else having or had this much trouble?

I might add, that even after replacing the booster and master cylinders, neither jeep has ever had super, what I would call quality brake action. I have always had to push a little hard on the peddle even with replacement parts.

I have never had any trouble bleeding the brakes all around, so I never really suspected a serious problem with the proportioning valves.

I am starting to wonder if rebuilt master cylinders just are not what they use to be.

I've had alot of problems from autozone rebuilt master cylinders, but I have lifetime warranties and they are the closest parts store. Always make sure to bench bleed well.

If you are losing fluid from the rear make sure to check the wheel cylinders closely. They are cheap and easy to rebuild if your not sure.

The best brake mod I have done was converting my Renix to a WJ booster/mc combo. It only took a few hours and I can lock down 35's with ease.

I think he meant he was losing fluid from the rear of the master cylinder, in turn damaging the booster.

My experiences are that the wheel cylinders have a tendancy to leak fluid after a few years. This contaminates the break shoes resulting in poor rear breaks.

The opposite is also true, the piston in the wheel cylinders tend to stick/freeze and do not move the shoes when the breaks are applied. You should remove the break drums and inspect the cylinders.

The propotioning valve ain't worth a dam after a few years. I removed the rubber "O" ring in mine and locked up my other wise non functioning rear breaks before the front will lock (this was rather dangerous). I simply unscrew the nut at the end, removed the spring and piston, remover the "O" ring and re-assemblied. I did not have to bleed it. I replaced the propotionong valve with an adjustable valve now that I have rear disks.

Blaine B. said:

I think he meant he was losing fluid from the rear of the master cylinder, in turn damaging the booster.

Click to expand...

Correct.

To help keep your posts on target, note that I had already replaced brake lines and hoses, wheel cylinders, adjusters, springs, front pads, rear shoes, drums and rotors with all new parts, and bleed everything, including myself, LOL.

The brakes have worked reasonably well at times, but even the new adjusters are a pain to keep set ( seems to be Jeep XJ brake problem). I intend to remove them and lube the threads with antiseize next time I get in there, but that is not my current topic or problem.

Anyone ever found a good inexpensive replacement for the proportioning valves?