Strange brake problem

My brakes have some kind of a problem. It feels kind of like a bad master cylinder so I have replaced the master cylinder twice now and the symptoms have not changed at all. After changing the MC the first time and it not fixing the problem I figured I must have bought a bad MC so I got another one (different brand) and there was still no change, so now I’m thinking it never was the MC and that there must be some other explanation. The symptoms I am having are that the pedal seems to catch at different places every time I press it. The first time I press it after it has been sitting, sometimes one of the back wheels will grab and skid a bit but it seems like the other wheels are not stopping. The rest or the time the peddle just seems kinda low and not always in the same place. It hasn’t gotten any worse or changed in the month or so that I have been driving it to work 3 miles (spare me the lecture I know I shouldn’t drive it) or so like this.

[font=&quot]Could a problem with the proportioning valve cause this kind of thing? I don’t know. Any ideas would be welcomed. Thanks[/font]

Did you "bench bleed " either MC?
Also need to know what year were talkin bout here,ABS,no ABS,front disc,rear drum 4wheel disc yadda yadda yadda
There have been alot of issues with brake boosters on pre 96 XJs.
96 and up have a dual diaphgram as oppased to the single diaphgram on the earlier models.The dual setup improves the stock braking,but lifts,bigger tires
etc will affect the braking

I would start with bleeding all the lines again and again. Then check the front callipers for a stuck piston. If its still not any of those, check the drum actuator on the rear and again see if one of them is stuck open or closed. I would also check the pads in the drum for proper adjustment, maybe one is cranked way aout while one is way in causing it to be more sensitive to braking.

The rear brake grab is a very common problem, with a number of putative causes, often hard to pin down. I'd start by opening up the rear drums and checking for a broken spring or other damage, and especially for seized adjusters and broken adjuster cables. I have found that my brakes often grab when one of them stops adjusting, and recover when I manually adjust them.

Check the frt. hub brgs. to see if there is any looseness. If the brg. is loose and the wheel/rotor are allowed to camber in as you drive, the pads are pushed back into the caliper. Next time you step on the brakes, it will take a lot of pedal travel to get the pads and rotor squared up and grabbing like they should. Pumping the pedal more than once will bring the pedal back up to normal height, and over short distances you may feel the pedal is normal. Turning or driving farther will allow the pads to retract again, and same problem again. Depending on side loads and how far it can feel different every time. Also, the rr. shoes if not adjusted properly can require a lot of pedal travel also before the shoes make firm contact w/ the drum. And a rr. shaft thats worn severely where the axle brg. rides could ptentially cause this same effect and also allow gear lube onto the brake surfaces which can cause erratic grabbing or poor braking.

Wow thanks for all the ideas. I was kinda expecting “that seems strange try replacing the mc again” it’s an 87 so no ABS and it has 33’s on it so braking has never been the best but at least it was the same every time.

[font=&quot]Explorer you suggested that a lose ft baring may cause this problem the hub is good but the ball joint got spread fairly bad from a resent broken u joint I wonder if that could be the issue? Also I guess the back brakes are due for some work. Thanks everyone![/font]

So did you replace the shaft when that u-joint broke? If so go back and retighten the nut on the stub shaft where it comes through the brg. Actually even if you didn't, tighten that nut w/ an impact or big breaker bar. The cases where I saw this cause the problem your describing where actaully good brgs. that just were not tightened properly causing the brg. to have slight play. Tighten them up and the problem is gone. The ball joints shouldn't really affect the braking other than causing some wandering problems.

explorer said:

So did you replace the shaft when that u-joint broke? If so go back and retighten the nut on the stub shaft where it comes through the brg. Actually even if you didn't, tighten that nut w/ an impact or big breaker bar. The cases where I saw this cause the problem your describing where actaully good brgs. that just were not tightened properly causing the brg. to have slight play. Tighten them up and the problem is gone. The ball joints shouldn't really affect the braking other than causing some wandering problems.

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[font=&quot]I could definitely give that a try. I did change the shaft. And I put the nut back on with a standard two-foot breaker bar not a torque wrench. I don’t know what they are supposed to be torqued to.[/font]

Off the top of my head, @180 ft./lbs. I think. I never torqued them, just as tight as my impact could get them. You are holding two halves of the inner race together, so within the capabilities of just about any 1/2" gun you aren't going to overtorque it.

explorer said:

So did you replace the shaft when that u-joint broke? If so go back and retighten the nut on the stub shaft where it comes through the brg. Actually even if you didn't, tighten that nut w/ an impact or big breaker bar. The cases where I saw this cause the problem your describing where actaully good brgs. that just were not tightened properly causing the brg. to have slight play. Tighten them up and the problem is gone. The ball joints shouldn't really affect the braking other than causing some wandering problems.

Click to expand...

[font=&quot]It worked just like you said. I tightened it up and the first time and it was quite lose. I started putting the cotter pin back in it popped real loud, so I pulled the pin back out and put the wrench back on and it was lose again so I tightened it again. I figure that pop was the barring popping back together. The breaking was much improved. I think after driving on it a bit I’ll try tightening it once more. Thanks for your expert help! You are the man.[/font]