I am having trouble with my right rear passenger brake locking up while I am driving,I was having trouble with the right front passenger wheel but this seems to have cured itself.This is the only wheel that it is happening to now.I have bled the system replaced the rear wheel cylinders and adjusted the porportioning vavle as the repair manual shows.The brake pads and the brake shoes are in like new condition.None of this has helped at all.I ran a diagnostic test on it and all comes back fine.
This does not always happen but more intermittently then anything,it does seem to be happening more now then before and also seems to be getting worse.

 

Have you had the drum turned or at least had it checked to make certain it isn't warped or "out of round"? If you have done all you say, thats about all thats left to check.

Make certain the spring assembly on the wheel in question is an exact mirror image of the opposite wheel. Also check the adjuster "star" wheel. If the teeth are worn badly or the metal tab that retains the position of the adjuster wheel is worn badly, the pads may be "adjusting up" too quickly.

Just a side note to consider too. These trucks are known for having the rear wheels grab or lock in the first few miles of operation after a severe change in humidity levels in the air (i.e. thunderstorm). Its a result of flash rust on the inner surface of the drum due to the high humidity. The condition will subside after a mile or so and a couple of normal stops.

 

I am not sure which valve exactly is the proportioning valve, but I was having a somewhat similar problem with my 90 except the front calipers kept dragging, and the problem turned out to be the valve that is located on the driver's side front frame rail. I have heard that called a proportioning valve, and also just called an ABS valve and I could not tell the proper name from seeing the box. But installing it fixed all my problems. It was an expensive dealer part though. And I did have both brake lights coming on frequently before replacing the valve but they have never done it since replacement.

I don't know if this helps you in any way, but that's what happened to mine. And I have read that those valves can be a real PITA as far as bleeding etc . There is a very specific way that it has to be done and if it isn't done exactly in the right sequence, it will give problems. I did not do the install on it so I don't know. I kept bringing the truck back for warranty problems with new pads and rotors, and it went back about three times before they figured out that the valve was causing all of the problems, so I told them to go ahead and replace it if they were sure that it would cure all of the problems, and it did. It cost me some time but it also cost them a lot of warranty labor time too, and three sets of new brake pads including a free upgrade to ceramics. So hopefully this info might help you get to the heart of the problem.

 

Did your shoes get any brake fluid impregnated into them. You replaced the wheel cylinders and said the pads/shoes were in like new condition. Once you check the runout on the drums, make sure that you don't have a liquid contamination issue in the shoes.

 

hey about your brakes lockin up could be of two reasons either you rear brake 's are too tight or too loose on that one wheel see when you step on the pedal too loose would cause it to snap out to the drum and cause instant lock up or if they are too tight the brakes are alraedy dragging and causing instant lock up
or quite possibly your e-brake is hanging up and causing it to be too tight i have had the same problem and it does not do it any more
>>>>>>>>and if you feel the pedal is too low with the brakes loosened up you need new drums just in case that is why you have them a lil tighter