Proportional pneumatic actuator with 11 sq. in. (71 cm2) effective area used to control damper and air valves in heating, ventilating and air conditioning systems. The type of positioning, proportional or two-position, is determined by the controller, such as a thermostat or relay. Actuators are positioned by air pressure acting upon a diaphragm and piston. Opposing the force on the top of the piston is a spring. The piston force overcomes the spring force, as the air pressure increases, extending the actuator shaft if the pressure lessens, the spring force retracts the shaft. Units are constructed of a sturdy die cast aluminum housing enclosing an easily replaceable beaded molded neoprene diaphragm, steel piston, piston shaft, sintered bronze shaft-guide bushing and steel spring. The molded neoprene diaphragm has a circular bead which mates with a groove in the pressure housing insuring positive locating and sealing of the diaphragm with the housing.