Disc Brake System: Description and Operation

When the brakes are applied, fluid pressure behind the caliper piston(s)
increases. Pressure is exerted equally against the bottom of the piston(s) and also against the bottom of the piston bore(s). The pressure applied to the piston(s) is sent to the inner brake pad, forcing the brake pad against the inner brake rotor surface. The pressure applied to the bottom of the piston bore(s) forces the caliper to slide towards the inner side, or toward the center of the vehicle. Since the caliper is one piece, this movement toward the vehicle causes the outer section of the caliper to apply pressure against the back of the outer brake pad, forcing the brake pad against the outer brake rotor surface. As line pressure builds up, the brake pads are pressed against the brake rotor surfaces with increased force, bringing the vehicle to a stop. When the brake pedal is released, line pressure is released and the piston seal(s)
causes the piston(s) to retract slightly. This results in less drag on the brake rotor by both brake pads.

The outward movement of the piston(s) and inward movement of the caliper
automatically compensates for brake pad wear. As the brake pads wear, the
increased area behind the piston(s) is filled with brake fluid from the master cylinder reservoir.