Brake calipers all work under the same premise. A hydraulic piston extends from the caliper bore when demand is placed on the brakes. The movement of the piston squeezes the friction material of the brake pads against the metal surface of the rotor. When replacing brakes, the pistons must be retracted to make room for the thicker pads and/or rotors. Not all caliper pistons employ the same procedure to retract the pistons, however. Rear calipers can sometimes require the piston to be turned clockwise (or counterclockwise in some cases) to retract the piston. Knowing which type of caliper you intend to use the disc brake piston tool on will help prevent damage to the caliper or tool.
Support the removed caliper to the suspension or frame of the vehicle with a bungee cord so it does not hang from the rubber brake hose. This will prevent damage to the brake hose. If applicable, remove the pads from the caliper. Some vehicles use pads that remain in the caliper mount and removing them from the caliper is not required.
Read the repair manual for your specific vehicle to determine which type of caliper you are trying to compress the piston to. Some pistons simply retract by slowly squeezing the piston into the bore. This can be done with the disc brake caliper piston tool set or even a large C-clamp. Place the wedge plate from the set onto the drive arbor of the tool. Insert the bottom of the drive arbor into the caliper piston. You can also use the backing plate of the old pad or a block of wood against the caliper piston. Because some pistons are phenolic, they can crack or break under duress. Place the wedge plate up against the outer caliper housing and tighten the tool until the drive arbor and wedge plate are working together to compress the piston. Continue to slowly tighten the tool by hand until the caliper piston is fully retracted into the bore. Loosen the tool and then remove it.
Match up the correct adaptor of the disc brake piston tool or the disc brake piston tool set to the screw-in type caliper. These pistons have visible grooves on the face of the piston. Many makes and models of cars employ different-size grooves that require different adaptors. The tool has six sides or the kit features up to nine applications. Insert the adaptor onto the face of the caliper piston until the notches mate the grooves. If using the set to retract the piston, employ the same procedure as in step two with the wedge plate and drive arbor up against the adaptor on the face of the piston. If using just the tool, a ratchet and extension with suitable matching drives will be required. Turn the ratchet or drive arbor clockwise to slowly compress the piston into the bore until it is fully seated. Make sure that the piston does not employ a left-handed thread. Although rare, some cars may use a right-hand thread piston on the left side and a left-handed piston on the right side. On some import cars, bottoming the piston fully into the bore can cause damage to the piston or parking brake mechanism. The manual will alert you of this if applicable. Using a ratchet may also require someone to hold the caliper steady for you while you start to turn the tool.
Inspect the caliper boots once the pistons are retracted and the tools are removed. Sometimes air can become trapped inside the boots when retracting caliper pistons. This will cause them to bubble up above the surface of the compressed piston and can get trapped between the piston and pad when the caliper piston returns. This will damage the protective boot. Lift up on the bubbled areas of the boot to release the air or use a plastic pick (not a metal one) to help you manoeuvre the boot to restore it to its collapsed position.
Before compressing caliper pistons, it's a good idea to remove half of the brake fluid from the master cylinder. This will prevent overflowing of the master cylinder when the fluid is pushed back into the cylinder. Some technicians prefer to open a bleeder screw on the caliper piston, especially when the brakes are integrated with ABS systems. Opening the bleeder screw prevents the fluid in the piston from backing up into the cylinder when the piston is being retracted. Any time a bleeder screw is opened, bleed the brakes to purge any air allowed into the system. Some vehicles employ dual pistons per caliper. There are some tool sets that supply an adaptor to retract the pistons together.
Any time caliper pistons are retracted, they need to be restored by pumping the brake pedal. This will apply hydraulic pressure back onto the compressed piston and seat them against the brake pads and restore the braking response. Failure to perform this task will result in complete braking power loss until the pistons are fully extended.