There's two possible causes besides the calipers, although the calipers are the logical first suspect. Start by raising the front tires off the ground to find which one is stuck. Open the bleeder screw. If there's a little spurt of fluid and the brake releases, check for brake fluid contamination by inspecting the rubber seal under the master cylinder reservoir cap. If it's blown up and mushy, the fluid is contaminated with petroleum product. All parts with rubber and / or seals must be replaced at the same time, and the steel lines must be flushed and dried. The lip seals in the master cylinder expand and grow large enough to block the ports in the master cylinder. Brake fluid can't release and flow back nto the reservoir so as it heats up and expands, it applies the brakes harder the longer you drive.
If there is no sign of fluid contamination, look at the front brake hoses. If there is a metal anchor bracket in the middle, there could be rust buildup inside where it's crimped around the hose. That will constrict the hose and prevent fluid from flowing freely through it. You can peel the crimp open a little with a large pliers. When this is the cause of the problem, the brake pedal will feel higher and harder than normal. With enough foot pressure, fluid will be forced past the restriction and apply the brake, but it won't be able to flow back.
Monday, February 15th, 2010 AT 1:05 AM