the diaphragm 58 are additive and act to the left as viewed in fig3 to exert a force on the plunger 72 to thereby maintain the valve member 74 in a fluid sealing engagement with the valve seat 76 the compound axial force generated by the spring 88 and the pressure in the left hand pressure chamber 60 acting on the left hand side of the diaphragm 58 are also additive and act to the right to oppose the compound axial force generated by the spring 100 and the pressure in the pressure chamber 62 referring now to fig2 for an aid in a detailed description of the actuator mechanism 28 it can be seen that the mechanism 28 comprises a tubular housing 120 the opposite ends of which are enclosed by caps 122 and 124 the cap 122 defines the outlet port 32 over which the conduit 34 is secured additionally the cap 122 has a longitudinal bore 126 which is reduced in cross section at its inner end to define a restricted passageway that communicates with the interior of the tubular member 120 the restricted passageway defines a small bleed orifice 128 which is protected from contamination by a filter 130 and a filter screen.