Abstract: A gas driven pump has a housing, first and second cylinders disposed within the housing, and first and second interconnected pistons having product intake and product exhaust positions disposed within the first and second cylinders, respectively. A slide valve has first and second positions for alternately pressurizing one of the first and second cylinders and venting the other of the first and second cylinders. A linkage moves the slide valve between the first and second positions thereof in response to movement of the first and second pistons. Over-center construction of the linkage prevents stalling of the slide valve, so as to assure reliable operation of the gas driven pump. The slide valve comprises a slide for placing an inlet port in fluid communication with an alternating one of two pressure/vent ports such that the slide valve scrapes frost build up from the pressure/vent ports as it moves thereover, thereby further enhancing reliability of the gas driven pump.