Questions and Answers
Drifting Brake Pedal
Q. I have a question concerning brakes. I have a 1989 GMC pick-up 2wd, ½ ton, regular cab, 5.7 liter 350 engine, not heavy duty, auto trans. My problem is with the brake pedal. It slowly goes down with steady pressure. My problem started with the pedal going all the way to the floor very easily.
I had a loud hissing of air from under the dash like a brake booster leaking, so I replaced it. The problem got a little better, so I thought the main seal in the master cylinder had gone out, so I replaced it as well. Prior to replacing the master cylinder I noticed my right rear wheel cylinder had a small leak in it so I replaced both the right and the left.
My calipers are less than a year old. I replaced my shoes because of the brake fluid from the leaking cylinder. I have just about replaced everything except the brake lines on this system that I know of. Is there anything else that could be causing the pedal to creep down? It does it when the brakes are cold and when they are hot. Could the isolation dump valve be bad? If so how could I check it? Also I have bled the system very well, so I pretty sure there is no air in the lines. Please help!!!!
Thank you very much,
NelsonA. You didn't mention that this truck has rear wheel ABS so for the moment I'm going to assume it doesn't.
What I would suggest is to clamp off the front brake hoses leading to the calipers and feel the pedal. If the pedal gets rock hard with no fade, then the front calipers are the problem. For some reason, and don't ask me what it is, GM calipers can cause this problem. I have seen it fairly often and a lot of good master cylinders get replaced because of it.
If you do have ABS, I would check the calipers as mentioned above. If the pedal still drifts down, then you might have a problem in the ABS Dump Valve. This was a problem with any truck that used the Wagner rear ABS system.
GM issued a bulletin on this and I quote:
"Some of the above mentioned vehicles may experience a "false" or dropping pedal condition that may feel very similar to a master cylinder with an excessive amount of by-pass. Upon installation of the replacement master cylinder, there is little or no improvement in pedal feel or height, This condition may be caused by the dump valve in the E-H (Electro Hydraulic) valve being held slightly open by a small particle of foreign material. This allows fluid to flow into the accumulator and create the low or dropping brake pedal."
There is a test procedure for this and I would recommend taking it to a GMC dealer to have it checked.
Information provided courtesy of ALLDATA
