Compressed Air Brake System

Compressed air brake systems are typically used on heavy trucks and buses (Note the difference between pneumatic brakes and pneumatic/hydraulic). The system consists of service brakes, parking brakes, a control pedal, an engine-driven air compressor and a compressed air storage tank. For the parking brake, there is a disc or drum brake arrangement which is designed to be held in the 'applied' position by spring pressure. Air pressure must be produced to release these "spring brake" parking brakes. For the service brakes (the ones used while driving for slowing or stopping) to be applied, the brake pedal is pushed, routing the air under pressure (approx 100-125psi) to the brake chamber, causing the brake to reduce wheel rotation speed. Most types of truck air brakes are drum units, though there is an increasing trend towards the use of disc brakes in this application. The air compressor air draws filtered air from the atmosphere and forces it into high-pressure reservoirs at around 120 PSI. Most heavy vehicles have a gauge within the driver's view, indicating the availability of air pressure for safe vehicle operation, often including warning tones or lights. Setting of the parking/emergency brake releases the pressurized air pressure in the lines between the compressed air storage tank and the brakes, thus actuating the (spring brake) parking braking hardware. An air pressure failure at any point would apply full spring brake pressure immediately.

In the Florida CDL Handbook , this process is described. Here is the section describing the service brake:

5.1.7 - The Brake Pedal

Brakes are applied by pushing down the brake pedal. (It is also called the foot valve or treadle valve.) Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes won't work.

These large vehicles also have an emergency brake system, in which the compressed air holds back a mechanical force (usually a spring) which will otherwise engage the brakes.Hence, if air pressure is lost for any reason, the brakes will engage and bring the vehicle to a stop.