23 with a sleeve nut 28 screwed onto an outer thread 27 of the displacement piston 23 the sleeve nut 28 has a central bore 29 through which the vacuum line 17 protrudes coaxially with the sleeve nut 28 and the annular disc 25 as far as the valve plate 21 thus a further valve opening 47 is formed between the annular disc 25 and the vacuum line 17 a valve closing spring 30 is supported at one end on the side of the valve plate 21 remote from the further valve seat 25 and at the other end on the bottom of the blind bore 24 the blind bore 24 communicates with the atmosphere via a ventilation conduit 31 at one end a valve opening spring 46 having a very much greater restoring force as compared with the valve closing spring 30 is supported on the displacement piston 23 and on the opposing inner wall of the valve interior 14 this valve opening spring 46 urges the displacement piston 23 such as to lift the valve plate 21 from the valve opening 19 on the other end the displacement piston 23 is actuated by a differential pressure transducer 34 to this end the displacement piston 23.