The metering device for a liquid medium comprises a cylinder and a piston operatively disposed therein to define a metering chamber ahead of the piston. The outlet end of the cylinder is open and closed by an evacuation valve. The piston is driven forwardly through a working stroke to force the liquid medium in the metering chamber out of the chamber and past the evacuation valve. A check valve located in the metering chamber moves from a closed to an open position on each working stroke to preclude entrapment of air in the metering chamber and thus provide for more accurate metering. The metering device may include means to regulate the length of the working stroke.