a device for expressing milk from one or more breasts has at least one , and preferably two expressers and a pump . the expressers each have a cup for fitting on the breasts and a cup outlet to which a vacuum created by the pump can be connected . the cups have a flexible liner which collapses around the breast to establish the vacuum . pulsating pressure is applied to the outside of the liner to increase the vacuum and stimulate milk production . the pressure is applied through lines which are vented to maintain adequate , consistent pressure pulses without stalling the pump .