A portable pumping device for drawing milk from a human breast includes a breast shield adapted to fit over a nipple of a breast and a flow line coupled to the breast shield. The flow line is adapted to allow air to flow there through a pump coupled to the breast shield via the flow line. The pump includes a pump intake and a pump exhaust and is operable to create a pressure drop between the nipple and the pump, wherein the pressure drop creates a suction at the breast shield by lowering the pressure of air in the flow line. A blow-back valve is disposed between the flow line and the pump. The blow-back valve has a valve piston disposed in a valve housing The valve housing includes a flow line aperture that communicates via the flow line with the breast shield, a valve inlet adapted to communicate external to the flow line, and a valve exhaust adapted to communicate external to the flow line. The valve piston is adapted to alternatively seal the valve inlet and the valve exhaust.