The air brake system is actually three separate systems :-
SERVICE BRAKE: This system applies and releases the brakes when the driver pushes/releases the brake pedal. Pushing the brake pedal, opens a valve to let air flow from the air tank through the airlines to a brake chamber.
PARKING BRAKE : Parking brake releases air from the brake chamber which allows the brake spring to expand and forces the pushrod out which pushes a slack adjuster that turns the camshaft, twisting the S-Cam and forcing the brake linings against the brake drum.
EMERGENCY BRAKE: When there is insufficient air in the system to keep the spring in the chamber restrained, the emergency brakes automatically engage. When air pressure falls below 420 kpa, a low pressure warning light will come on along with an audible buzzer. If air pressure continues to fall, the emergency brake will automatically apply when air pressure drops to 140-310 kpa. The fail safe engineering of air brakes will not allow you to control activation of the emergency brake so look for a safe place to pull off the roadway as soon as the low pressure warning activates.

sos