A piston inside the cylinder then applies pressure to a liquid called brake fluid. This pressure is transmitted through tubes called brake lines to apply the brakes. Meanwhile, speed sensors send electric signals representing the wheel speeds to an electronic controller. If a wheel begins to lock, sensor signals cause the controller to signal a pressure-modulating valve assembly. The assembly then applies pulses of pressure to this wheel's brake until the wheel rotates normally.
Hydraulic disk brakes use fluid pressure to force nonrotating brake pads inward against the disk.