Questions and Answers

Weird GMC Brake Problem

Q. I have rebuilt some of the power brake system on this vehicle, 1988 GMC 1500 Vandura Van. In November I noticed the left wheel cylinder was not working and leaking fluid.

I replaced the rear wheel cylinders on both rear wheels then proceeded to replace the brake shoes. I also had a mechanic replace the front calipers and disk brake pads on the front of the vehicle. He also replaced the master cylinder. After driving the vehicle for about a week, in cold weather, the left rear wheel locked up after I parked the vehicle. I replaced the adjuster and retaining springs. The new left rear wheel cylinder was leaking fluid. I bleed the brakes again and replaced the brake cylinders with AC Delco cylinders from the dealer. I drove the vehicle for about a week only to experience the same problem. I rotated the rear wheel drums, adjusted the brakes and continued to have the same problems after about a week.

The only parts I have not replaced are the power booster and the combination valve. The brake lines are also the original brake lines. I checked them for leaks and kinks. I found none and the brake fluid travels well when bleeding the brakes. The brake light indicator on the dash indicates no problem. I have power brakes no ABS. I hope you can shed some light on the subject. I like this van and do not want to part with it. It is also fuel injected and performs well. The rear end , transmission and components have been replaced or rebuilt. It still has the original engine with about 153,000 miles on it. The rear brake drums have also been replaced.

Sincerely,
Umoja

A. This is a real stumper. I can't think of anything that would cause one wheel cylinder to leak like that after a week or so. You seem to have done everything the way it should be done.

There are only two things I can think of here that might be causing the problem. One is you may have a defective master cylinder and the other is the rod from the vacuum booster to the master cylinder needs to be adjusted. To get the rod adjusted correctly, you will need to take it to a GMC dealer. There is a special tool to insure the rod is adjusted correctly. But I would try replacing the master cylinder with another new one before I do that.

It is possible the proportioning valve is not working properly. There are two hydraulic circuits in the van, one is for the LF and RR and the other is for the RF and LR. This is a safety feature. If one circuit should fail, the other would still work. If it is allowing too much pressure to go to the LR/RF, then it is possible the extra pressure will blow out the wheel cylinder. The caliper being so much heavier would be able to withstand the extra pressure.

That's about the best I can do for you. For whatever it's worth, I hope it helps. Please, let me know what you find with it.