of the nozzle is tied into the working chamber 12 in the region of the mounting of the base member 7 2 in the region of the mounting of the base member 7 2 of the switchover piston 7 the wall of the housing module 20 3 which forms part of the working chamber 12 has an axially extending groove 10 so that in the position of the switchover piston 7 shown there is a fluidic connection between the outflow channel 4 and the inflow channel 3 opened up by the valve closure member after the switchover operation has occurred the groove 10 ensures a corresponding fluidic connection between the inflow channel 2 and the outflow channel 4 the two annular grooves 9 of the frustom shaped valve closure members 7 2 are each lined with an o ring 8 1 8 2 in accordance with a selected switchover pressure the position of the switchover piston 7 shown in fig2 is obtained when the inflow channel shown on the right of the drawing cf position 3 in fig1 is connected to a gas container and the second inflow channel remains free this piston position is also obtained when a corresponding gas container with substantially equal internal pressure is connected to the second inflow channel cf position 2 in fig1 when the switchover piston 7 is pushed to the left in the plane of the drawing with the inflow channel 3 open the entire outer surface of the valve closure member 7 2 on the right of the drawing the end face of the pin 7 3 and the circular area of the part of the o ring 8 1 projecting from the wall of the frustum cf position f 1 in fig1 is acted upon by the.