Disc brakes are also self-adjusting. As the pads wear, the pistons compensate by moving closer to the disc. Each time the brakes are applied, the pistons travel until the pads contact the disc. As the pad wear progresses, the pistons slightly override the deflection of the rubber seal, but the piston is retracted each time only by the amount of the deflection. The space behind each piston increases with pad wear, and is filled with extra braking fluid from the reservoir.