The purpose of a Vacuum Priming Valve is to automatically allow air to be drawn out of the pumping system until the pump fills with water. Then, when the water reaches the priming valve, the float rises and closes the priming valve to prevent fluid from flowing to the vacuum priming system. The priming valve will continue to release air while the pump is running.
Vacuum Priming Air Valves
For more information on the Vacuum Priming Air Valves