Abstract: A refrigerant transfer system is provided embodying a pump for controlling the level of liquid refrigerant within a suction header of an evaporator unit. The pump includes a pair of elongated chambers in endwise relation. One chamber has a substantially greater cross-sectional area than the second chamber. Opposite ends of each chamber have port means. A first set of inlet and outlet valve means is connected to the port means at each end of said one chamber. Each inlet valve means is alternately connected to a source of pressurized fluid. A second set of inlet and outlet valve means is connected to the port means at each end of the second chamber. Each inlet valve means of the second set communicates with a lower portion of the evaporator suction header. The heads of a double-headed piston are disposed within the chambers. Control means are operatively connected to the first set of inlet valve means for controlling the direction of movement of the piston heads within the chambers.