Calipers are very important components in disc braking systems. Pistons expand from hydraulic pressure on demand when the brake pedal in the vehicle is applied. The pistons squeeze the brake pads, which then contact the surface of the rotor and slow the vehicle down. When a caliper becomes defective, there will be some obvious telltale side effects. Identifying the symptoms will help you find the bad component and then replace it.
Open the bonnet and check the fluid level of the master cylinder. A leaking piston, bleeder or brake hose will drain the reserve fluid from the master cylinder eventually. If necessary, top the master cylinder off with clean brake fluid.
Test drive the vehicle to begin diagnosing the calipers. Choose a car park or low-traffic road if possible. Before taking off, be sure you have a firm brake pedal. If the brake pedal fades to the floor with a poor braking response, do not test drive the vehicle.
Bring the vehicle up to 30mph and then apply the brakes firmly. Determine whether the vehicle pulls to the right or the left when the brakes are applied. This would be a good indication that a caliper piston is sticking and only one side of the brakes are working properly.
Drive the vehicle for 10 to 15 minutes applying the brakes often. Bring the vehicle back to the area where you plan on disassembling it for further diagnosis. Before beginning, place you bare hand near each wheel that employs calipers. Do not touch the wheel because a stuck piston will heat the brakes and nearby components up severely and you could burn your hand. Burning brakes will give off both intense heat and a pungent burning odour. If a wheel is indeed extremely hot, allow it to cool down before proceeding.
Lift the axle of the vehicle that contains the suspect caliper. Use the jack to lift it and be sure to place it safely onto jack stands.
Remove the wheel using a lug wrench to remove the wheel nuts.
Remove the two caliper guide bolts using a ratchet and a suitable socket.
Pry the caliper off of the rotor with a small pry bar. Some calipers will contain pads attached to the caliper while others will leave the pads behind in the caliper anchor.
Inspect the caliper for any visible signs of fluid leaks. Leaking brake fluid will be very obvious. Thoroughly inspect the piston area, the bleeder screw and the brake hose connection. Also inspect the rubber protective boot surrounding the pistons. Tears or rips in the boot will not necessarily mean the caliper is defective, but it will compromise the pistons eventually. While you're at it, inspect the brake hose for any visible signs of cracking or rips in the rubber.
Hang the caliper to the suspension using a caliper hook or a wire hanger. Compress the piston using a C-clamp or a caliper piston reset tool. Some rear disc-brake calipers will require the reset tool to screw the piston into the bore while most all front calipers will require compressing the piston into the bore with a C-clamp. Compress it slowly. If the piston does not bottom out in the bore or resists compression, the piston is bad and the caliper needs to be rebuilt or replaced.
If there are no signs of leaking brake fluid and the caliper pistons compress well, the calipers are OK. Check the slides and the pads next. Some vehicles are notorious for sticking caliper slides or pads that get stuck in the bridge. These two symptoms will give off similar evidence that the caliper is faulty. Removing the sticking slides or the stuck pads and relubricating the anchor or slides are all that is usually required and the caliper can be reused.