a breast pump includes a platform resting on the floor or other support and a treadle having a convex surface resting on the platform , permitting a pivoting or see - saw motion of the treadle . a bellows is connected between the treadle and the platform , forming a variable displacement chamber . a flexible hose connects the chamber to a reservoir including a container , such as a baby bottle , and an adaptor attached to the bottle as a cap . there is no valve controlling the flow of air to or from the reservoir , thereby providing complete and precise control of the pressure in the reservoir .