Quoting: CHP
Brake Bias
Q: I have been having a problem with my '89 IROC-Z's rear brakes. The car is a four-wheel disc brake system from the factory. The brakes just don't seem to hold well at all in the rear. 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, so I have literally replaced every component on the brake system except 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 Chevy dealer and a Midas center to fix, with no resolution. 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 in it. It never bottoms out on the floor, and when I take the caliper off, the pedal pushes the pad out about 3/8 inch with one push of the pedal. I also had this problem on an '88 Z28. I am at wit's end! Any help would be greatly appreciated.
Mike Huelsman
Mentor, OH
A: First of all, I think that you're running into a couple of things. Disc brakes are not a self-energizing braking system; they only have the force that the caliper can apply at any given time. Drum brakes have the rotation of the brake drum to force the brake shoe into the drum. The hydraulic force from the wheel cylinder pushes the primary (front) shoe into the drum, and the shoe is rotated, putting pressure into the secondary shoe and into the reaction pin on the backing plate. This is why you have so much holding force with drum brakes. That is why most "foot brake" drag racers will use drum rear brakes, so they can hold the car on the starting line at a higher rpm against the converter. With disc brakes, the rotors and pads must be up to operating temperature before you will have any braking force at a dead stop. Yes, you could increase the line pressure (install an adjustable proportioning valve) to the rear brakes to aid your dead stop issue. The only problem I see with this is when you get the brakes up to operating temperature, you will have way too much rear brake bias and the rear brakes could lock during spirited driving. When going into a turn, that's the last thing you want to happen.
If you would like to do a quick test to confirm this, drive the car and get the brakes up to temperature. Bring it home and immediately jack up the rear of the car and redo your braking test. The rear brakes will hold once they have some heat in them. Also performance (i.e., racing) pads will only aggravate the problem. Most performance-compounded pads need more temperature to create the proper coefficient of friction to perform properly.
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