At least one valve (2) has a flat valve seat (15) with an annular opening (8). It has an annular flat valve disc (12) which closes off the opening and is held against the valve seat by a spring element (13). The valve disc is located in a flow chamber (11) adjacent to the annular opening and the walls of which encircle the valve disc. The face of the central opening (14) in the annular valve disc is slightly smaller or the same as the cross-sectional face (27) between the outer edge of the valve disc and the walls of the flow chamber.
