in which the measuring piston moves and a separate air passageway to that air chamber controlled by a spring biased check valve through which compressed air is delivered to the pipette reservoir during a discharge stroke application_number US 89512778 A description reference is now made to fig1 wherein a cylindrical tubular barrel 10 is shown provided with a nozzle member 11 at its forward end and a bonnet member 12 at its other end these latter members are threaded onto barrel 10 the connecting end of nozzle member 11 is internally threaded to cooperate with the external thread provided on barrel 10 while bonnet member 12 is externally threaded for connection to the internal thread provided at that end of the barrel bonnet 12 has a central bore that is threaded preferably with a multi lead thread for a purpose that later will be disclosed an o ring piston seal 13 is provided as shown at the forward end of barrel 10 so that the threaded connection between the barrel and nozzle member 11 becomes air tight when the two parts are threaded together moreover a metering piston seal 14 is mounted between the end face of barrel 10 and a shoulder formed on the internal bore in nozzle member 11 a flanged washer 15 and a wave washer 16 are provided as shown a piston 17 is provided for longitudinal movement within barrel 10 and the bore of nozzle member 11 the piston 17 comprises a main body 20 that is provided with an air passageway 21 and a radial aperture 22 leading from passageway 21 to the outer peripheral surface of piston 17 it will be observed that piston 17 is smaller in.