Questions and Answers

Chevy ½ Ton Low Rear Brake Pressure Q. I have a 1991 Chevy ½ Ton 4x4 with a soft brake pedal. It is a 350 engine, automatic transmission, fuel injection, rear ABS, A/C, cruise and regular duty brakes and suspension 10" drums and 6200 GVW. I have owned it since new and never had any success in troubleshooting the problem. There is 140,000 miles on the truck.

I feel the biggest symptom is the lack of wear on the rear brakes. The originals would still be on the truck if I hadn't changed the shoes to better quality to try to improve stopping power, they were not even ½ worn at 100,000 miles. I also notice when bleeding the brakes, lots of very tiny bubbles circulate back through the fluid reservoir (have not allowed the reservoir to get low).

These bubbles also work their way through the system when bleeding. The adjusters appear to work, they are easy to turn and the lever makes good contact. I have manually adjusted them several times and the brake peddle is restored for a period of time. It takes about two full turns on the adjusters, the drums are snug to get on, without force and no drag when turning them after installed.

I have bled down the brakes several times and this helps, including changing the fluid. I have also had the master cylinder replaced, without any notable change to brake performance. I have just installed the third set of pads. Hard braking numerous times in reverse does not seem to help get the adjusters to set the brakes up better.

In short, it "seems" there is insufficient brake pressure to the rear brakes, either for normal stopping or when backing up to activate the rear adjusters enough. The brake fluid also gets within six months, lots of very fine black particles suspended in it (no feel of solids when rubbing between fingers).

There has never been any warning lights for the brake system either. The truck has been at the local GM deal several times (they say everything is normal) and several independent mechanics. The service bulletins have been searched without any help.

A couple of problems have been suggested. One is that the replacement master cylinder was faulty, or there is a problem with the rear ABS controller. The ABS controller is an expensive fix.

Thank you for your assistance...

A. Basically the truck stops but your not happy with the performance. Has anyone tried to spin the rear wheels with the truck in the air? Hit the pedal and see if both sides apply?

On occasion I've seen some rear wheel cylinders seize up or not apply fully. This also should have RWAL, Rear wheel anti-lock brakes. It's possible that the dump valve is restricted or the base brake combination valve (proportioning) is not letting enough fluid to the rear wheels.

Also could be the rubber hose at the rear axle joint is partially collapsed. With this mileage all hoses should be replaced anyway. that could explain the black particles in the fluid.