Brake calipers apply hydraulic pressure to the brake pads. The pads squeeze against the rotor on each side and slow the vehicle. This is much the same way caliper brakes work on a bicycle. When you activate the cable-driven hand brake, the caliper squeezes the pads on either side of the tire rim and slows the bicycle. The difference between the two is hydraulic pressure. There are a few reasons why a brake caliper can stop functioning the way it was intended to. Checking the caliper for performance issues is recommended when your brakes aren't working properly.
Lift and support the vehicle using a jack and jack stand(s). Be sure to loosen the lug nuts with a lug wrench before lifting and supporting the vehicle.2
Inspect the brake hose and caliper for visible signs of brake fluid leaks. Brake hoses with visible cracks or deterioration should be replaced immediately. Brake fluid seeping or leaking from the caliper piston may or may not be visible until the caliper is removed. An excessive leak will be obvious and the caliper should be replaced.3
Place the C-clamp over the caliper so the bottom drive of the C-clamp sits against the outboard pad and the top of the clamp sits over the caliper housing. Tighten the clamp in order to compress the caliper piston. If the piston does not retract or gives excessive resistance, the piston is compromised and requires replacement. When checking rear calipers, be aware that some require screwing in the piston which requires a special tool kit in order to achieve. Trying to compress a screw-in piston with a C-clamp or like device can damage the piston or clamp or both. Screw-in piston type calipers are only used on some rear disc brake systems.4
Move the caliper with your hands once the piston is retracted. Free-floating calipers employ slides that need to move back and forth. The slides may be separate or integrated with the caliper bolts. It is not uncommon for the slide or slide bolts to corrode and prevent the caliper from moving properly. To fix the problem the caliper would need to be removed by removing the caliper guide bolts or slide bolts. The bolts would require cleaning and re-lubricating or replaced and lubricated. Internal slides can also be maintained in this fashion.5
Remove the pads and compare the pad wear from both. One pad wearing more than the other may indicate a sticking piston or sticking slide. Common side effects in this case include the vehicle pulling to one side when the brake pedal is applied. If the slide or piston is beyond reconditioning, replace the caliper and then bleed the brake system6
Bleed the brake system any time the hydraulic system is exposed to air. Brake crimps placed on the brake hoses will prevent too much air from entering the system, but bleeding will still be required. To bleed the brake incorporate the assistance of someone to pump the foot brake pedal four to five times and hold it in the down position. Open the bleeder screw on the caliper and allow air to purge from the brake lines. Strategically place a drip pan below the caliper to catch the brake fluid. Close the bleeder screw while the pedal is still in the down position and then repeat until the pedal feels firm. Add more brake fluid to the master cylinder to prevent it from running dry when bleeding brakes. Also, return the cap or cover to the master cylinder before allowing the assistant to pump the brake pedal again.