I have a problem with a 1985 Chevy G30 van with the hydroboost system.The brakes are locking up from time to time and also are grabby at times. I know that the front brakes are locking up but I am not sure yet about the rears. If I let it set for a while the pressure bleeds of and they may be OK for a week or more. I have bled the front brakes to remove any dirt. Opening the bleeders does not to seem to instantly release the brakes either. The brake pedal does not seem to be sticking and the actuating rod length has never been changed.I have been told the master cylinder will not cause this. What about the hydro booster or proportioning valve? The brake system warning light DOES NOT come on when this happens.I am desperate here as this is the truck I use to make my living as a contractor.If anyone has experienced this please contact me.
I don't know a whole bunch about these systems, but I can tell you that the combination (AKA proportioning) valve, if it has/sees issues, will turn on. But it is designed for a brake bias issue between front and rear.if one or the other circuits drops pressure, the brake light will come on. Also, if the master cylinder leaks internally, the brake light should come on. Assuming brake light works, you need to test the combo valve to make sure it is working properly. All you do is locate it (on trucks it was located on the furthest forward crossmember, just follow the two brake lines from the master to it) and pull the single wire connector off of it. With the key in run, as I recall the circuit, the light should stay off. Now ground that wire to a clean spot on the frame or engine. The light should come on. That means the circuit is working, and IF the proportioning valve was faulty, the brake light should come on, which essentially eliminates it as a cause of the problem. Sorry I can't help beyond that, but I don't know how the hydroboost applies braking effort.almost sounds like a stuck valve somewhere.
