Do-it-yourselfers often use a C-clamp to retract the piston or pistons. In this case, we simply levered the piston back using a 2x4 and a piece of plywood. By doing this, the brake fluid in the pistons is being pushed back into the master cylinder reservoir through tiny passages, so the pistons move slowly. The width of the 2x4 allows both pistons to be pushed in at once. If you pushed in one by itself, the other would pop out — you don't want that. Fortunately, most cars have just one piston per caliper, which makes things far simpler. Either way, steady pressure and patience are key here. In this example, we added a second plywood shim near the end of the process to fill the ever-increasing gap. Throughout, take the utmost care to ensure you don't nick or tear the rubber boot and seal that encircles the pistons.