Pneumatic air brakes are used everyday in trucks, buses, and tractor-trailers. Air brakes are divided into two systems, the supply system and the control system. The supply system stores compressed air and supplies it to the control system by a belt or off the engine timing gears. The control system is split into front and rear wheel circuits. The service brakes are applied by an air valve powered brake pedal, which engages the brakes. When the parking brake is engaged, the spring force from the spring brake cylinder is released by compressed air, and in turn engages the parking brake. The air used for engaging the brakes is re-circulated in to a reservoir where the air is eventually reused. These brakes are used everyday, and are extremely important.

Image via Wikipedia