Abstract: A rocker valve for alternately sealing one of two outlet ports is provided with two cylinders in which there are respectively disposed pistons carrying sealing members for longitudinal reciprocation along the extended axes of valve seats respectively surrounding the port openings. The rocker is rotated to urge one piston toward its respective valve seat while permitting the other piston to be retracted from its respective valve seat. An elastomeric diaphragm is mounted between the sealing members mounted on the pistons and the valve seats whereby the sealing members urge the diaphragm against the valve seats during sealing. The longitudinal motion of the pistons prevents skewing of the sealing members during rotation of the rocker arm for maintaining the sealing members parallel to the valve seats throughout the strokes of the pistons.