piston rod 12 in the area behind the valve housing 33 here the piston rod 12 has an annular groove 58 with a graduated circle cross section is inserted the piston 21 is provided on its open side opposite the annular bottom 31 in the non compressed state with a bore 60 extending coaxially to the axis 2 the shape of the piston 21 in this area in the non worked state is shown in fig2 with a line of dashes and double periods after assembling the valve and the piston in the manner described above the piston is compressed in the area of this bore 60 in the direction of the piston rod 12 it then axially adjoins the securing ring 59 and in its exterior end area the piston rod with its tapered section 61 in the course of this compression the piston 21 is axially stressed against the valve 22 the seal 41 being axially compressed between the annular bottom 31 and the annular collar 40 because the ring 42 of the seal 41 is rigid per se only the relatively thin sealing faces 62 63 which abut on the annular bottom 31 or the annular collar 40 can be compressed in the uncompressed state these sealing faces 62 63 have a thickness of a few tenths of a millimeter e g 0 3 to 0 6 mm to allow deformation the piston 21 consists of a.