Abstract: Double action piston pumps incorporate a check valve mechanism to control material flow between the pumping chambers and the pump outlet. The present disclosure includes an improved check valve mechanism which is mounted in the pump in free floating relation with the pump piston to permit more efficient closing of the check valve and therefore provide more efficient and consistent output flows. This check valve has an inner annular surface surrounding the pump piston and is separated therefrom by a clearance of between .035 and .085 inches. When the check valve is closed, during part of the pumping cycle, it seats on an improved valve plate thereby preventing a reversal of flow. The valve plate on which the check is seated is in a close dimensional tolerance with the piston rod to effectively scrape the piston rod clean thereby preventing the adhesion of the material being pumped to the piston rod.