Abstract: An automatic liquid pump contained within a sealed pressure vessel. The pressure vessel includes a liquid inlet opening, a liquid outlet opening, a gas inlet and a gas outlet. A preloaded float-operated over-center valve mechanism closes the gas inlet valve and opens the gas outlet valve to permit vapor to vent through the gas outlet valve. The over-center valve mechanism alternately closes the gas outlet valve and opens the gas inlet valve to admit compressed gas into the sealed vessel to evacuate the liquid within the sealed vessel through the liquid outlet opening. A float valve is coupled to the liquid outlet opening for closing the liquid outlet opening when the liquid in the vessel drops below a first predetermined level. As a result, vapors are prevented from escaping through the liquid outlet opening under all operating conditions.