drain port 32 formed therein a spacer 33 and sleeves 34 35 are mounted in the housing 27 slidably mounted in the sleeves 34 and 35 are a spool 36 and a piston 37 respectively formed in the plug 25 is a cylinder 38 in which a movable stopper 39 is inserted the movable stopper 39 can be moved or adjusted by means of an adjusting screw 40 threadably engaged with the plug 25 a spring 42 is interposed between a spring retainer 39a of the movable stopper 39 and a spring retainer 41 of the spool 36 the spool 36 has formed therein a passage 45 which communicates the spring chamber 43 with a chamber 44 defined between the spool 36 and the piston 37 the chamber 44 communicating with the drain port 32 the piston 37 has a large diameter portion 37a and a small diameter portion 37b a chamber 46 defined between the portions 37a and 37b communicates with the port 31 and a chamber 47 formed in the rear part of the small diameter portion 37b communicates with the port 29 further the port 5 of the two stage proportional pressure reducing valve 2 communicates through a conduit 48 with the port 29 of the negative proportional pressure reducing valve 3.