What's the symptom? Does the pedal go to the floor or is it high and hard but the car won't stop? If the pedal goes to the floor, the master cylinder may have been damaged. Typically the pistons are pushed part way back into the calipers to make it easier to reassemble them. You have to stroke the brake pedal a few times to work the pistons back out. Most people push the pedal all the way down, but experienced professionals never push it more than half way. Crud and corrosion build up in the bottoms of the bores where the pistons don't normally travel. When the pedal gets pushed too far, the seals can get ripped on that crud.
There is a related problem that only occurs on GM front-wheel-drive cars. When the pedal is pushed too far, a valve trips in the master cylinder when there are unequal pressures in the two hydraulic circuits. That happens when there's a leak in a line but it also happens when one caliper pushes its piston out and starts to build pressure before the other side does. When that valve trips, you will not be able to force any brake fluid from one front caliper and the opposite rear brake. That typically does not cause a low pedal; in fact, it can go unnoticed until the pads are worn down on one side and still look like new on the other side.
Oct 28, 2012 at 1:43 AM
