Of course, the hydraulic system does not stop the car all by itself. In fact, it really just transmits the action of the driver foot on the brake pedal out to the wheels. In the wheels, sets of friction pads are forced against rotors or drums to slow their turning and bring the car to a stop. Mechanical force (the driver stepping on the brake pedal) is changed into hydraulic pressure, which is changed back into mechanical force (brake shoes and disc pads contacting the drums and rotors). The amount of force acting on the friction pads and shoes is equivalent to the psi applied to the pedal multiplied by the area of the piston affected. A force of 25 psi applied to the pedal times 4 square inches of piston area equals 100 pounds of pressure in the system.