bore adjoins the channel 35 according to fig4 and 6 a and receives a twin valve unit comprised of a first lower valve body 36 and a second upper valve body 38 this stepped bore has a guide section 39 adjoining the channel 35 wherein a valve chamber 41 adjoins the guide section while forming a valve seat 40 by means of an additional valve seat 42 a valve chamber 43 adjoins the valve chamber 41 wherein the valve chamber 43 is connected by means of a valve seat 44 to the valve chamber 45 into which a venting channel 46 opens laterally the valve body 36 has a valve shaft 36 1 guided in the guide bore 39 which has about its circumference several axial slots 36 2 on the topside of the valve shaft 36 1 a sealing ring 36 3 is provided which in the rest position is forced by the restoring spring 36 4 supported between the lower and upper valve bodies 36 38 against the valve seat 40 the valve body 38 has a valve shaft 38 1 guided in the valve chamber 41 which is essentially configured as a hollow cylinder with lateral wall openings 38 2 and whose interior is in communication with the valve chamber 41 this valve body 38 supports a first lower sealing ring 38 3 for cooperation with the valve seat 42 as well as a second upper sealing ring 38 4 for cooperation with the valve seat 44 the valve body 38 is also provided with a piston 38.