When I start a brake job I use a flat-blade screwdriver to gently pry the piston back into the caliper before I unbolt it. If it goes in fairly easily with one hand, it is okay. If you have to really struggle but it still doesn't move, open the bleeder screw, then try it again. If that works, suspect the brake fluid is contaminated with a petroleum product like engine oil or power steering fluid. That makes all the rubber parts that contact the fluid swell and expand. The lip seals in the master cylinder will grow past the fluid return ports and block them. The only proper repair for that is to remove every part that contains rubber, flush and dry the steel lines, then install all new rubber parts. If any one rubber part is not replaced, the contamination will leach out of it and recontaminate all the new parts.
When the piston will not go in by hand, even with the bleeder screw open, it's time to suspect dirt or rust buildup on the piston. That will make it stick on the rubber square-cut seal. Even if you do get the piston to go in far enough to allow installation of new pads, it's going to stick when you apply the brakes. Under light pedal pressure the piston won't move. Under heavy pedal pressure the piston will move out to apply that brake, then it won't release. If the resulting dragging is bad enough, the heat can melt plastic wheel covers.
You'll see some people use a c-clamp to push the piston into the caliper after it has been removed. If that is the only way to get it to move, you have junk. We used to rebuild every caliper with every brake job, but today calipers cause such little trouble, and the cost of professionally-rebuilt calipers has come down so much that you can't justify the time needed to clean and rebuild them yourself. If you do replace them, many are available with plastic or chrome plated steel pistons. Both should have the same type of pistons so they heat up evenly. Plastic pistons are less likely to transfer heat from the brake pads into the brake fluid. Steel pistons can absorb more heat so they take longer to heat up, and they cool down slower.
Thursday, January 30th, 2014 AT 12:10 AM