a pneumatic reciprocating mechanism comprising a working member reciprocably powered in a housing and forming together with a vibration absorbing reaction support member and an elastic seal element a working chamber which is supplied with pressure via an air feed valve . the latter is positively coupled to the working member and controls the air flow through openings in the reaction support member in response to the relative positions of the working member and the reaction support member . the elastic seal element is associated with the working member and has an annular valve collar which is arranged to establish sealing contact with a diverging peripheral wall of the reaction support member by radial expansion due to the air pressure in the working member .