against or nearly against the pump cover 26 at this point the fluid medium to be pumped is located in the pumping chamber 22 and air at ambient pressure is located in the pump activating chamber 24 the discharge stroke begins when the air valve 110 is opened and air pressure is allowed to enter the pump activating chamber 24 the air pressure pushes the diaphragm membrane 30 down thereby expelling the fluid medium contained in the pumping chamber 22 into the fluid transporting duct 40 because the inlet check valve 46 will not allow the fluid medium to go through the inlet 42 the fluid medium is forced through the outlet check valve 48 once the discharge stroke is completed the air valve 110 is closed to allow the compressed air in the pump activating chamber 24 to escape and the pressure is returned to atmosphere it should be noted that the air valve 110 is a 3 way valve it is normally closed in its unenergized state in its open position air is allowed to pass from conduit 164 to conduit 166 in its closed position air is allowed to pass from conduit 166 to the atmosphere the fill stroke of the diaphragm pump 10 begins with the diaphragm membrane 30 returning to.