a pumping system for delivering unstable fluids under pressure , comprising a pair of alternately operated single - acting pumps operated by a double - acting air motor . the air motor is controlled by a reversible spool valve and mechanically operated air pilot valves . the single - acting pumps deliver and contain relatively small quantities of the unstable fluid to a positive flow metering system for connection to an outlet to spray guns or other equipment using the fluid . the metering system includes a metering valve , a flow meter and a bypass valve for removing trapped air and presetting flow .