encloses a motor not shown which is drivingly connected to an output shaft 11 formed with a drive square at its rear end the housing 10 is provided with an inlet passage 12 for motive air see fig1 a supply valve 13 for controlling the motive air flow is provided in the inlet passage 12 and comprises a valve body 14 which is shiftable by means of a lever 15 the valve body 14 has an enlarged head portion 16 fig2 a and 2b which sealingly cooperates with an annular seat 17 the seat 17 is formed by an elastic sealing element 18 coaxially with the supply valve 13 the housing 10 is provided with a cup formed valve housing 19 having a cylindrical bore 20 within the bore 20 a tube shaped valve body 21 is movably guided at its opening direction facing end hereinafter called its rear end the valve body 21 is formed with an annular end portion 22 which is arranged to sealingly cooperate with a seat 23 at the end wall of the valve housing 19 as the valve body is in its open position similarly at its closing direction facing end hereinafter called its forward end the valve body 21 is provided with an annular end portion 24 which is arranged to sealingly cooperate with a seat 25 formed by the elastic sealing element 18 the diameter of the bore 20 exceeds the diameter of the valve body 21 to form an annular clearance 26 around the valve body 21 the clearance.