the bore 38 downwardly so as to be directly adjacent the valve chamber member 35 of spherical configuration and seated on valve seat 34 thereof the valve closure member 35 equally is loaded by a spring 49 disposed in sleeve 42 which spring is supportd on an inwardly protruding flange 50 of sleeve 42 the inlet connection 9 terminates in an inlet chamber 51 confined by the larger front face 52 of the stepped piston 37 similarly the inlet connection 15 terminates in an inlet chamber 53 confined by the largest sized front face 54 of the stepped piston 40 the smaller sized front face 55 of the stepped piston 40 faces the smallst sized front face 56 of the stepped piston 37 with both front faces 55 and 56 confining an outlet chamber 57 in communication with the outlet connection 26 and being loaded by a common compression spring 58 the stroke of the stepped piston 37 is on the one hand confined by a step 59 of the housing bore 31 on the part of the larger sized front face 52 of the stepped piston 37 and on the other hand by a step 60 of the housing bore 31 respectively placed into abutment.