A positive displacement pump used for lifting small quantities of water and discharging them under low heads. It has a plunger arm operating either on an eccentric shaft or a rocker arm thrusting on a rubber diaphragm stretched over a cylinder. As the diaphragm is depressed, the water and air in the cylinder are forced out through the discharge side of the pump. As the diaphragm is lifted, a vacuum is created in the cylinder, and water is forced in.