If the hoses are the problem, the caliper will release when you open the bleeder screw, but it will not release when you crack open the steel line at the master cylinder. If it doesn't release, the piston is sticking in the caliper. GM doesn't really have much trouble with that. Of course you have to crack open the system while the brake is still locked so you can watch for it releasing. The hoses are a more common problem on Chrysler products and there is a real easy fix. They use a metal bracket in the center of the hose to hold it in position. Rust builds up inside where that bracket is crimped around the hose. Eventually it constricts the hose to the point you can force fluid to the caliper with the brake pedal, but the fluid won't travel back to the master cylinder right away. Generally those will release after a few miles, but not before they get nice and hot. The fix is to just peel the crimp open a little with a large pliers.
If the caliper does release when you crack the line at the master cylinder, suspect fluid contaminated with petroleum product such as engine oil, power steering fluid, or ball joint grease from your fingers. That will make rubber seals swell and grow past the return ports in the master cylinder. The only proper repair for that is to replace every part in the hydraulic system that has rubber parts. It's a very expensive repair.
One word of warning on GM brakes. When you bleed them, never, never, ever push the brake pedal to the floor. That applies to any car with a master cylinder that is more than a few years old. Crud and corrosion builds up in the lower halves of the bores where the pistons don't normally travel. When you pedal bleed the system and push it down all the way, you run the risk of ripping the lip seals on that crud. At that point the master cylinder is junk. In most cases a rebuilt unit is less expensive than the kit to rebuild it yourself, plus it's properly cleaned and you get a warranty.
What's more important on just GM vehicles is when you pedal bleed, there is a valve in the master cylinder that trips to block fluid flow to what it thinks is a leak. That valve trips when there is a difference in pressure between the two hydraulic circuits. We have found no way to reset that valve other than to give it a short burst of compressed air into the opened bleeder screw of the caliper that is not flowing fluid, then let it gravity-bleed. Once fluid is flowing from both calipers, close the bleeders, very slowly press the pedal repeatedly no more than 1/4 way to the floor to push the pistons out, then crack the bleeders once more to remove the last few bubbles of air. The symptom is one caliper and the opposite rear brake will not flow any fluid. The secondary symptom is the pads on the working caliper will wear out quickly and the other side will not wear at all. You would expect to have a horrendous brake pull, but because of these split-diagonal hydraulic systems on front-wheel-drive cars, the engineers have modified a non-adjustable alignment angle called "scrub radius" to overcome the effects of that brake pull. A lot of people don't even know they have a braking problem until they hear grinding noises from the working side
caradiodoc
Thursday, December 2nd, 2010 AT 6:01 PM
