How to Retract Piston in Rear Disc Brake
When doing repair work such as pad or rotor replacement on rear disc brakes, one of the most common problems amateur mechanics run into is retracting the caliper piston in order to fit the installed pads over the rotor so that the caliper can be re-mounted to the hub housing. This can be a very frustrating problem and is often the end of what could be a worry-free home brake job. Here are the basic steps for retracting the caliper piston while doing a brake job.
Have the Right Tools
Beyond the basic tools that you need for basic auto maintenance, you’ll need a few extra things while working on your disc brakes. One is a medium size “C” clamp. For most vehicles, a 6 inch clamp is plenty big enough, however an 8 inch clamp may be necessary for very big vehicles with the largest brakes. It’s a good idea to have a can of special non-chlorinated brake cleaner on hand in case you have to clear off excessive soil. You’ll also want a few clean, thick rags to use as well.
Get the Caliper Ready
Once the vehicle is raised and the wheel is removed, select the correct tool to loosen the caliper mounting bolts. Keep in mind that you may need Torx® head wrenches or other special tools. Once the caliper mounting bolts are removed, the caliper itself should easily slide free of the rotor. Once the caliper is removed from the hub, you will be able to remove the pads relatively easily as well. Keep in mind that even though the mounting bolts have been removed from the caliper, it will still be attached to the vehicle by the brake hydraulic line. This should not be removed or loosened for any reason. Now you’re ready to compress the piston.
Compressing the Piston
Once the caliper is removed, the piston should be pretty easy to spot: it’s the circular part on the inside surface of the “C” shape of the caliper. It is normally shaped like an empty can. The first thing you should do is inspect the condition of the piston. Look at the seal surrounding the piston’s outer surface to make sure that it there aren’t any signs of leaking hydraulic fluid. Clear excessive soil from the area surrounding the piston if necessary and clean the outer surface of the piston itself. Next, place one of the rags inside the cup of the piston and slip the clamp around the outer surface of the caliper, with the adjustable side of the clamp resting inside the piston cavity.
Once the clamp is in place, simply tighten it slowly, taking care that the piston is compressing straight back in the bore, and not being forced at an angle. It should move back pretty readily with the clamp pressure, and eventually it will reach the end of its range when it is fully retracted in the caliper housing.
Now that the piston is retracted, you are ready to install your new pads or rotors, and complete your brake job.