A motorized pump is disclosed that includes a flexible diaphragm fitting within a rigid member, a motor drive mechanism for drawing a puller member attached to the diaphragm away from the rigid member to create a space between the diaphragm and the rigid member and form a negative pressure region within that space, and an outlet communicating with the negative pressure region. In one embodiment, the pump provides a negative pressure in a single chamber which can be used to operate one or two breast shield assembles. In another embodiment, the pump is particularly adapted for double-pumping and provides two chambers which generate vacuum.