Abstract: A control valve with a three part, all plastic housing having an access opening through which a slide extends for operating the plastic valve element toward and away from the valve seat, the plastic valve element having an integrally molded plastic diaphragm clamped between parts of the all plastic housing and traversing the entire access opening, the slide being operated by a rotor, there being ramp shaped surfaces on confronting faces of the rotor and slide, and an adjustment nut for locating the rotor, slide and valve element relative to the valve seat.