A pumping system includes a probe to suction a fluid from a fluid reservoir, a pump in fluid communication with the probe, and a sensor for detecting phase changes in said pumping system. The sensor is in fluid communication with the probe or pump and is operable to generate a sensor signal. The pumping system also includes a fluid exit from the pumping system that is in fluid communication with said pump, and a variable force check valve that is located between the probe and fluid exit.