diaphragm 52 in sealing contact with annular valve seat 42a the requirement for and advantage of this sealing contact in the valve in its assembled form are hereinafter described diaphragm valve 52 is made of a resilient material which will withstand contact with the high or low temperature fluids in the water system and is the type of valve described in u s pat no 3 495 803 the features of the diaphragm valve will be hereinafter described so as to explain the function of the invention but reference is made to the aforementioned patent for a more complete description of the construction of the diaphragm valve and the pilot valve pin valve 52 has central portion 52a with a central opening through which pilot valve pin 34 exceeds central portion 52a also has a plurality of radial openings 52b extending from enlarged portion 52c of the central opening the function of radial openings 52b and enlarged portion 52c will be hereinafter described central portion 52a extends upwardly and sealingly contacts depending annular portion 50b of retainer 50 relatively thin connecting portion 52d connects central portion 52a with movable sealing portion 52e which sealingly contacts annular valve seat 42a connecting portion 52d allows sealing portion 52e to move away from valve seat 42a to open the valve by means which will be hereinafter described to describe the operation of the air vent device it will first be assumed that it is connected to a functioning water system and that all parts are in the position shown in fig1 and 3 sealing portion 52e of the valve 52 is maintained against valve seat 42a by action of air pressure in rear chamber.