A belt drive from the engine drives the air compressor. The compressor is connected to a storage reservoir that stores the pressurised air. The whole system is controlled by a governor, which keeps the pressure at a set level. When the air pressure reaches 125 psi, the air compressor governor stops the compressor from pumping in any additional air. If the pressure drops to 100 psi, the governor turns the air compressor pump back on again.
When the driver pushes on the brake pedal, it directs pressurised air through hoses into the braking mechanism. Most trucks use foundation brakes. In foundation brakes, the pressurised air pushes out a piston which moves a cam, pushing a brake shoe into a rotating drum. The drum is attached to a wheel and when the brake shoe rubs against it, it creates friction, slowing the wheel down. When the brakes are released, some pressurised air is released from the system and the compressor has to store new air to keep the brakes powered.
Trucks also have a backup system called spring brakes. If the compressor should fail or the braking system should begin to leak, it will make the service brakes inoperable. If that happens, the spring brakes will automatically stop the truck. Spring brakes are powered by a heavy-duty spring mechanism inside the brake chamber. This mechanism is normally held back by the air pressure in the system. If that air pressure drops too low, the spring brake system will engage, activating the brake drums. When the driver puts on the parking brake, the spring brakes are what holds the truck in place, preventing it from rolling away.