A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. To stop a car, the brakes have to get rid of that kinetic energy. They do so by using the force of friction to convert that kinetic energy into heat. When you press your foot down on the brake pedal, a connected lever pushes a piston into the master cylinder, which is filled with hydraulic fluid. In vehicles with a conventional proportioning valve, the front brakes typically wear two to three times faster than the pads or shoes in the rear. Consequently, a vehicle may go through one or two sets of front pads before all four brakes need to be relined.