Abstract: This invention relates to a rotary valve device for controlling and distributing flow of a fluid medium directly in response to the rotary motion of a motor shaft. Fluid medium is continuously supplied to a cavitated disk through a manifold by a pump source where the cavitated disk rotates at the same rotational speed as the motor shaft. The cavitated disk has a first void region in continuous fluid communication with the pump source and alternately communicates with cylinder conduits to drive a piston. A second void region in the cavitated disk is so dimensioned and proportioned to permit fluid medium to be alternately exhausted from the cylinder and to be distributed to exhaust conduits and then to the atmosphere.