the piston 36 at the beginning and 36 u2032 at the end of a pump stroke fig4 b shows the elastically deformable sealing portion 25 which is fastened to the piston rod 23 by means 26 not drawn a part 27 of the piston rod 23 is sticking out of the sealing portion 25 support portion 28 is hanged up on ring 29 which is fastened to the piston rod 23 support portion 28 can turn around axis 30 loading portion 31 comprises a spring which is fastened in a hole 32 onto the piston rod 23 the sealing edge 38 fig4 c shows that part 27 of piston rod 23 is almost covered by the elastically deformed sealing means 25 u2032 which has now increased its length and decreased its diameter the sealing edge 38 u2032 the distance a u2032 between the sealing edge 38 and the central axis 19 of the chamber is approximately 40 of that of distance a in the shown transversal cross section fig4 d shows section a a of fig4 b the loading portion 31 is fastened at one end in the hole 32 of the piston rod 23 the support portion 28 and the ring 29 the support portion is stopped by a stop surface 33 not drawn the support portion 28 is guided by the guiding means 34 not drawn fig4 e shows section b b of fig4 c the support means 28 and the loading means 31 are moved towards the piston rod 23 the rib 22 fig4 f shows an alternative for the loading means 31 it comprises springs 35 on each axis 30 fig5 a b c d e f g h show details of the third embodiment it is a variant of the first embodiment the sealing portion.