the displacement of the auxiliary piston 9 this gets into the range of a groove 6 which is provided in the inside wall of the hollow piston rod 8 and which acts as bypass and through which the air can get round the auxiliary piston 9 and through the bore 5 to the suction side of the main piston although the reduced pressure at the suction side of the main piston is lowered hereby so that the auxiliary piston 9 again moves out of the range of the groove 6 the reduced pressure is however again increased on a further motion of the main piston so that the auxiliary piston again gets into the region of the groove with the consequence of the supply of further air to the suction side of the main piston 4 on completion of the checking process this entry of additional air leads thereto that the main piston again pressed back into the initial position by the compression spring displaces a greater volume than corresponds to its piston stroke it is made certain hereby that the liquid which has entered the suction duct 31 during the checking process is again pushed out with certainty furthermore air can get through a choke bore 6 39 at the end of the suction stroke of the main piston to its suction side the air entering through this choke bore 6 39 can also serve for the actuation of a whistle so that the arrangement of the auxiliary.