Rear Brakes Lock Up on dry and wet ground????

okay ...this problem has progressivly gotten worse the longer i own the jeep...its an 89 ...4.0.....3'' rustys sp.....31x10.5x15.....

sometimes (well....most of the time nowadays) when i BARELY press the brake pedal the rear wants to lock up and slide.......when we had everything apart doing the lift we checked the brakes out and the fluid and everything seemed fine...except for a bit of rust in the brake components in the drum assemble.....everything looked okay....well we put the lift on and for about a week or so it didnt do it......but now after wheelin on sat......its been really bad......EVERY time i BARELY hit the brakes the rear wants to slide.. :flamemad: ...what should i check first?????....i am slowly learning more about brakes..i just want to know where to start to save me buying all those expensive parts if its a quick fix...

thanks guys....

The GMC pickup I drove for work last summer was doing the same thing. A new master cylinder fixed it so you might wanna check that. Also maybe the porportioning valve.

Dean

Sounds as if you have either brake fluid (wheel cylinder) or gear oil (axle seal )contamination on your shoes. Even a small amount will cause instant lockup of the brakes. If you are clean in that area you may have a problem with the proportioning module (front/rear bias) on your brake system hydraulics.

If you do a search for brake locking, I think you'll find that this is a very common problem with XJ's, and there are about as many possible solutions as there are pieces in the brakes. But I've found that one consistent contributor is bad adjustment. The automatic adjusters don't work very well, and it seems that when one brake goes off a little, the leading edge digs in and causes that lockup. The most common complaint is that it will do this in damp weather, and sometimes it will improve briefly after a few applications, suggesting that it might also have to do with the lining material or drum surface.

Among other suspects: a bad wheel cylinder; bad springs; too deep a ridge on the drum, combined with axle play, causing edge of shoe to dig in intermittently; bad proportioning valve; your guess.

Some people have solved the problem simply by replacing everything. I usually get temporary relief by manually adjusting the brakes good and tight, but have yet to find a true "this is exactly it" answer.

Check your parking brake cable isn´t too tight. If the parking brake is too tight, this messes with the adjusters, can adjust up a bit too tight when backing up.
Like the others have said, can be many things, most times with mine, it was a sticky star wheel adjuster and/or a leaky wheel cylinder/brake fluid soaked shoes.
Isn´t a bad idea to carefully (they will tear when old) open a corner of the dust cover for your wheel cylinder periodically and see if any fluid has leaked.

Sorry to bring up an old topic, but my brakes did something similar today.

I went and got her muddy yesterday helpin a friend of mine, and took it up to the local do it yourself car wash place today to blast off the under carriage. Well I got the majority of it clean, and I had mud built up in my rims that was throwing it off balance, so I went to blast that out. Blasted around the rear drums and front pads, and got em good and clean. Well when i got back in and I got ready to leave, I put my foot on the brake and put it in gear, and everything was normal. Well I get to the next stop light and I start to stop, and the pedal drops to the floor and the rear end locks up and comes out from behind me. I managed to stop before I skidded too far out into the intersection, and once the cloud of smoke from my tires cleared, I thought all was good, and maybe the tires were just still wet and I hit some dirt on the ground or something. Well i get to the next light and dammit, same thing happens again, and this time i'm trying to slow down from a slightly faster speed (about 50mph). I managed to stop, and by now i've flat spotted my tires.

So when the light changed, I slowly crept into the parking lot of the first store I could find and my friend who was following me the whole time got out and we started looking underneath and couldn't find anything. Then I brought it to his house and put it up on some car ramps and still couldn't find anything. All the brake lines and such look good, and there's no leaks.

So now I turn to the experts. Master Cylinder shot???

99 sport, disc in the front, drums in the back.