Abstract: An annular sealing element (16) for sealing an annular gap between the inner surface of a cylindrical wall and a cylindrical piston which can be displaced axially relative to said wall and is arranged concentrically therewith, has two separate annular and axially adjoining parts (42, 44), of which the first part (42) consists of an elastomer material and the second part (44) consists of a rigid material. In order to ensure, in conjunction with a displaceable body of a valve, that the opening and closing behavior of the valve is as constant as possible, the two parts (42, 44) are connected to one another in a releasable manner, and the part (44) made of rigid material has an axial protrusion (46) which is formed integrally with it, runs around it in the form of a ring, passes more or less to the full extent through the part (42) made of elastomer material and has its end surface covered over to the full extent by the part (42) made of elastomer material.