The pistons in brake calipers are responsible for the brake pads making contact with the brake discs and consequently slowing the car, so they’re a vital component of the braking system.
When you put your foot on the brake pedal, brake fluid is forced from the master cylinder, down the brake pipes and into the brake caliper, where it pushes the pistons out behind the brake pads. This forces the pads to make contact with the discs.
When you take your foot off the brake pedal the pistons - and therefore the pads - retract, thanks to a drop in brake fluid pressure and the piston seals, which return to their uncompressed state.
Pistons are found in both floating and fixed brake calipers. There’s usually one or two large diameter pistons in a floating caliper, on the inboard side of the brake rotor. When activated, the piston(s) draw both pads toward the brake disc.
On fixed calipers, there are small-diameter pistons on both sides of the rotor, providing more braking force. Typically you’ll find four or six pistons per caliper. This is the type most commonly found in modern cars.
Brake caliper pistons can be made from plastic (phenolic), steel or aluminium. Aluminium is light and steel is strong, but aluminium can transmit too much heat to the brake fluid and cause brake fade, and both types of metal corrode.
Once they start to corrode they can stick in their bores, causing the brake pads to stick on or not be applied properly. Phenloic pistons are light and don’t conduct heat, so are useful in heavy braking environments such as motorsport.
Brake piston problems can be caused by corrosion or swelling (which causes the pistons to stick), a brake fluid leak (which means there isn’t enough fluid in the systems and the pistons can’t move as much as they should) or damaged piston seals.
When replacing brake pads a C-clamp is used to force the pistons back into the calipers.
Cars with rear disc brakes have self-adjusting pistons that move in and out when the parking brake is applied. These pistons usually screw in and out, so need to be turned to make them retract for pad replacement.