Brake Caliper Piston
The hydraulic piston inside a brake caliper that extends under fluid pressure to press the brake pads against the rotor.
The brake caliper piston is a cylindrical metal or phenolic component housed inside the brake caliper. Its function is to convert hydraulic fluid pressure from the master cylinder into mechanical force. When you step on the brake pedal, brake fluid is forced into the caliper, pushing the caliper piston outward.
The piston presses directly against the inner brake pad, clamping it (and the outer pad, via the caliper body) against the spinning rotor to slow the vehicle. Caliper pistons are fitted with a rubber seal to prevent fluid leaks and a rubber dust boot to keep out dirt and water.
Over time, the piston can corrode, or the dust boot can tear, allowing dirt to cause the piston to bind or seize inside the caliper bore. A seized caliper piston can cause the brakes to drag (overheating the rotor and pad) or prevent that wheel from braking entirely.