First, remove a cap on the master cylinder and look at the rubber seal. If it balloons up and is soft and mushy, the fluid has been contaminated with petroleum products. In that case, all rubber parts must be replaced. The seals in the master cylinder grow and block off the return port, so brake fluid gets trapped and the brakes can't release.
When the brakes lock up again, try loosening the two nuts attaching the master cylinder to the booster. If the car rolls freely, the booster push-rod is mis-adjusted, or more likely, the brake pedal is being held down slightly by a mis-adjusted brake light switch.
If only one front brake is locking, open the bleeder screw to that caliper. If the brake releases, suspect rust buildup inside the metal bracket in the center of the hose. As the rust builds, it pinches off the hose. The high pressure from your foot forces fluid to the caliper, but the fluid will not flow back to the reservoir. As the brake heats up, the trapped fluid expands so the brake applies harder, and the fluid gets hotter. A channel-lock pliers works great to peel the crimp open slightly on the metal bracket.
Sunday, April 5th, 2009 AT 11:28 AM