a breast shield adapted to fit over a nipple of a breast;

a flow line coupled to the breast shield, the flow line adapted to allow air to flow therethrough;

a pump module coupled to the breast shield via the flow line, a pump intake and a pump exhaust and being operable to create a pressure drop between the nipple and the pump module, wherein the pressure drop creates a suction at the breast shield by lowering the air pressure in the flow line; and

a blow-back valve disposed between the flow line and the pump intake and the pump exhaust, the blow-back valve constructed to alternatively cycle between connecting the pump intake to the breast shield, in a first position, in which the pump module draws air from the breast shield to generate suction to express breast milk, and to connect the pump exhaust to the breast shield, in a second position, in which the pump module pumps air toward the breast shield through the flow line,

wherein the blow-back valve has a valve piston disposed in a valve housing, wherein 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, wherein the valve piston is adapted to alternatively seal the valve inlet and the valve exhaust,

wherein the valve piston comprises an intake seal, an exhaust seal, and a valve plunger wherein the flow line aperture is disposed between the intake seal and the exhaust seal,

wherein the valve housing includes an intake sub-housing and an exhaust sub-housing, wherein the intake sub-housing is coupled to the pump intake via an intake line and adapted to accommodate the intake seal therein, and wherein the exhaust sub-housing is coupled to the pump exhaust via an exhaust line and adapted to accommodate the exhaust seal therein, and

wherein when the exhaust seal seals the valve exhaust from the pump exhaust, the pump intake is adapted to communicate with the valve inlet and the pump exhaust is adapted to communicate with the flow line aperture, said communication resulting in an over pressure between the pump and the breast shield.