sealably connected to a bellows 35 is provided within a housing 36 the diaphragm 34 is adapted to actuate a valve rod 37 and valve 33 the diaphragm 34 is biased in one direction by a spring 38 and biased in the other direction by compressor suction pressure a screw 39 is used for adjusting the tension on spring 38 lands 41 and 42 are provided on the rod 37 to alternately direct hydraulic fluid to and from the lines 26 and 27 as shown in fig1 the rotor 1 is being driven by the drive shaft 12 in the direction of the arrows the plate 17 is in its clockwise position the control valve 25 is in a position exhausting the housing 33 39 by means of exhaust line 31 to sump s because the suction pressure in the housing 36 is insufficient to overcome the bias of spring 38 and the land 41 is in a position to permit hydraulic fluid from the pump p by way of line 28 to enter line 26 fill the chamber 23 to move the protuberance or extension 21 to its shown position and the oil from the chamber 24 is exhausted through the hydraulic line 27 the valve housing 33 39 and the exhaust line 32.