A rotary valve rotated by an arm connected to a piston being driven by the pressurized fluid, either water or air. The orientation of the valve body in the valve housing will direct the fluid in one of two directions to move the piston. The arm of the rotary valve is connected to a cam that rotates a shaft connected to the valve body. Two cam rollers are biased towards the outer surface of the rotating cam member. The cam rollers provide a force to the cam to aid in the changing of the orientation of the valve body. There are two bump outs on the cam that correlate to the end of the piston movement so that the rotary valve does not stop at an end point of the piston stroke. The rotary valve may be is utilized in a chemical dosing or chemical application device that is driven by pressurized fluid.