When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.