Method and apparatus for pumping high viscosity fluids

A method and apparatus for pumping high viscosity paints. The apparatus comprises a double diaphragm pneumatic pump having means for controlling the air exhaust rate from the diaphragms to thereby reduce the diaphragm cycling rate. The decreased diaphragm cycling rate enables high viscosity paint to be drawn into the pump chambers and thereby avoids cavitation and increases the output flow rate. In a preferred embodiment, the bleed port, which vents air from the diaphragms to the atmosphere, is fitted with a needle valve, enabling the exhaust rate to be variably controlled. An alternative embodiment comprises a pump fitted with a valve received by the main exhaust of the air motor to thereby control the exhaust rate from the diaphragms.