1989 chevy astro rear brake failure?

Question

During an ice storm the rear wheels on my '89 Astro would keep turning despite appying brakes, very dangerous, the fronts work fine. The "brake" light on the dash is not on and there are no flash codes from RWAL computer. When raised onto jack stands, I can duplicate the condition by putting it into gear. I have to use the parking brake, or shut the engine off, to get them to stop before putting tranny back into park.

Mechanical inspection showed rear brakes in good shape and move when brakes are applied. Bleeding did not help. During bleeding, the brake light did not come on, but the light works when tested? Three choices appear to be master cylinder, proportioning valve or rear antilock valve. I would like to get it the correct one on the first try.
Thanks,
Steve

Answer 1

i own a repair shop ,and from my experience with those vans i have to say its probably the proportioning valve on it causing this to happen,i have had a few bad experiences with those before,and also master cylinders it seems like everything gm built from 88-92 had faulty master cylinders on them, i wind up changing a lot of those though ,they don't seem to apply the right amount of pressure to the rear brakes,that's about all i can tell you about those,good luck,i hope this help,s.

Answer 2

Let's work backwards and use some elimination processes, shall we?

If the brakes work with the parking brake, it's not a mechanical function of the pads that is the problem.

If it had anything to do with the antilock braking system, you'd have some time of indication from the ABS light or some other code being set.

If it were the master cylinder, you would have a complete braking system failure as opposed to the front working fine and the back not working at all.

That leave the only logical choice, the proportioning valve. That's what a prop valve does, it directs the proper pressures to both the front and back brakes. If you can get pressure at the front and not at the back, there's no doubt in my mind that it's the Prop valve.

Answer 3

Sounds like possibly a proportioning valve.. but also if you have not flushed the old fluid out of that old of a truck you should .. The fluid over time gets water contamination and can cause the cylinder to not have enough pressure. Also be sure that there is no oil ect on the shoes. I.E. leaking wheel bearing seal..

Answer 4

I would think the proportioning valve might be suspect as if it were a wheel sensor the ABS light would come on, I would try changing the proportioning valve first.