orifice allows air to be expelled from the system during the filling of a pipeline and admits air back into the system whenever sub-atmospheric pressure occurs. Air is vented from the system until water enters the valve and lifts the float against its seat, ensuring a tight seal. In the event of sub-atmospheric pressure in the system, the water level drops causing the float to fall from its seat and allow the admission of air. During the normal working of the main, the small orifice releases the air which accumulates under pressure. With the main in operation, the float is normally against its seat. As air enters the chamber body the water level is depressed until a level is reached when the float drops from its seat, which allows air to escape.