A difficult rear brake dilemma
I’ve been having a problem with the rear brakes on my 1989 Pontiac Firebird Formula. The car has a factory 4-wheel disc brake system and the rear brakes just don’t seem to hold well at all. If I jack the car off the ground and leave it in gear at idle, the wheels still turn. I was restoring the car from the engine back and have literally replaced every component on the brake system—all but the flex connectors to the back brakes at the wheels. I have even tried different brake pads from regular asbestos to Wagner race brand.
I have had the car to a Pontiac dealer and a Midas center with no solution. When I bleed the brakes, I get plenty of fluid to every wheel. The pedal always has a solid feel to it with no sponge. It never bottoms out on the floor and when I take the caliper off, the pedal pushes the pad out about 3/8” with one push of the pedal. I have also had this problem on a 1988 GTA. I am at my wit’s end!
This is a perplexing problem, particularly because you state that the pedal “always has a solid feel to it.”
A number of different defects can cause poor rear brake performance in a 4-wheel disc brake system but a telltale sign for most of the issues is a soft brake pedal. Taking this into account, I think the most likely cause of your poor rear brake performance may be defective rear brake hoses. If nothing else, the fact that you have already replaced “every component in the brake system” except for those hoses certainly makes them suspect. An obstruction in the hoses that’s preventing sufficient brake fluid volume and pressure from reaching the calipers would cause poor braking performance.
Another possible cause is a defective or incorrect master cylinder. For example, a master designed to work with a front disc/rear drum system delivers less fluid volume and pressure to the rear wheels compared with a master designed for a 4wheel disc setup. So if you install a front disc/rear drum master on a car with 4wheel disc brakes you will normally get poor braking performance in the rear.