An air operated double diaphragm pump comprising an air valve mechanism. The air valve mechanism may comprise a single valve system that replaces the two valve system found in conventional air operated double diaphragm pumps. The position of the valve may be determined by the diaphragm motion of the pump up to a stall position. A spring-loaded mechanical actuation may complete the valve action from the stall position to the final or end of stroke position of the pump.