The brake system uses friction to slow or stop a moving vehicle. It converts the vehicle's kinetic energy into heat energy. There are several types of brake systems including mechanical, disc, hydraulic, power-assisted, air, electric, and hand brakes. The hydraulic brake system works through pressurized fluid to actuate brake pads or shoes against drums or discs attached to the wheels. When the brake pedal is pressed, fluid pushes the pistons out to engage the brakes. Power-assisted brakes use vacuum assistance from the engine to increase braking power. Air brakes are commonly found on heavy vehicles and use compressed air to actuate the brakes.