by a hole 22 in rod 9 and a further hole 10 in rod 9 connected to hole 22 a compression spring opposes the system pressure in the pressure space 5 one end of the spring bearing against the annular zone at the end of the hole 10 and the other end thereof bearing against the rear wall of the housing 4 there are further pressure chambers 6 and 7 in the housing 4 to the left of the piston 8 one chamber 6 being connected with the tank 14 and the other chamber 7 being connected with the constant rate feed pump 2 a sleeve 16 is interference fitted into a cylindrical hole in the housing 4 in the vicinity of the pressure chambers 6 and 7 adjacent to the pressure chamber 7 the sleeve 16 has holes 17 and adjacent to the pressure chamber 6 it has slots 18 which are circumferentially distributed between a rear cylindrical part of the housing 4 and the circumferential surface of the piston rod 9 there is a longitudinally sliding spool 12 able to be borne either in the cylindrical hole or on the circumferential surface of the piston rod the seal 19 serves to provide a sealing action between spool 12 and the circumferential surface of the piston rod 9 the seal 20 provides a sealing action between spool 12 and the non interrupted parts of the sleeve 16 and the seal 28 acts on the rear cylindrical hole in the housing 4 between the front end wall of the spool 12 and the back side of the piston 8 a spring 13 is held compressed at the end part of the piston rod 9 there is an abutment 21 such as a retaining ring.