Abstract: The invention relates to a valve with a piston which is movable to and fro in a housing between an open and a closed position, wherein the piston has a first contact zone which in the closed position comes into contact with a second contact zone on the housing. The first contact zone on the piston is constructed as a cone which increases from a first diameter to a greater second diameter. The second contact zone on the housing is constructed as a flexible lip. The piston is configured in such a way and the flexible lip is deformable in such a way that--when the valve is installed--the piston can be pushed through the opening of the flexible lip in the region of the greater second diameter of the conical surface.