Questions and Answers
GMC Pickup Rear ABS Problem?
Q. Dear Vince, I own a 1988 GMC pickup 4x4 with front disc and ABS drum rear brakes. My rear brakes never worked since my truck was new, a fact I only found out about when I had my front pads changed. The fluid was pretty contaminated and dirty looking so I changed the wheel hoses also.
As I was bleeding the brakes my steel line that runs from the master cylinder to the rear brakes sprung a leak so I changed that line too. Refilled my master cylinder, and proceeded to bleed my brakes. Front brakes bleed fine, and pedal was pretty hard. When I bleed the rears- nothing! Just a few drops of fluid came out. It was the same on both sides.
Now I'm thinking a bad master cylinder? Or maybe bad wheel cylinders? Or possibly a bad proportioning valve? Or maybe even clogged bleeder screws? The rear brakes are almost new and the truck has 110,000 miles on it. Maybe you can give me some ideas on what might cause a situation like this?
Also there are no lights on dash to indicate a problem. I'm at the point were I just want to bypass the ABS system. Hell it hasn't worked in 12 years anyhow. As far as dealer codes are concerned, bringing it to a dealer would cost and arm and a leg. It's not worth it in my opinion. Lets see what you can help with. I thank you for any help you give me on this matter Vince.
Sincerely,
FrankA. First of all, those are rear wheel anti-lock brakes. There is probably nothing wrong in that side of the system. Have you checked for the wheels locking up while someone steps on brakes? The rear wheel cylinders could be seized up, but they should still bleed.
Most times when a line breaks and the pedal goes to floor, the combination valve slides to one side and turns on the red brake light. You may need to tap lightly on the combination valve and help it return to it's center position. Also try to leave the rear bleeders cracked and let it gravity bleed for a while as you keep an eye on fluid level in the master cylinder.
