A pump comprising a liquid housing configured with a liquid chamber having a single piston/diaphragm assembly comprising a spring and arranged between a second gas chamber and the liquid chamber, the single piston/diaphragm assembly drawing liquid from an liquid inlet fitting into the liquid chamber during a suction stroke and providing the liquid from the liquid chamber to an outlet fitting during a pressure stroke; wherein the spring is configured in the liquid chamber such that the spring stores energy during the pressure stroke and provides the suction stroke, the pump further comprising a slide valve assembly being movable between a pressure stroke state and a suction stroke state for alternately providing pressurized gas to the second gas chamber such that the liquid passes from the liquid chamber to the outlet fitting, and providing gas from second gas chamber to atmposphere, such that the spring provides the suction stroke and liquid is drawn from the liquid inlet fitting into the liquid chamber, characterized in that the the slide valve assembly comprises a block or housing assembly and an actuator assembly, the block or housing assembly comprises a slide valve housing (22a1) configured with a cavity; the actuator assembly comprises a slide block (22b1) configured to slide in the cavity of the slide valve housing (22a1), the actuator assembly having a slide spring (22b3) arranged between a lower retainer (22b2) and an upper retainer (22b4), and a yoke (22b5) configured with an opening to contain the actuator combination under compression, and further comprising a piston shaft/actuator slide assembly coupling the yoke (22b5) and the piston/diaphragm assembly causing the slide valve assembly to change between the pressure stroke state and the suction stroke state. a liquid housing configured with a liquid chamber having a single piston/diaphragm assembly comprising a spring and arranged between a second gas chamber and the liquid chamber, the single piston/diaphragm assembly drawing liquid from an liquid inlet fitting into the liquid chamber during a suction stroke and providing the liquid from the liquid chamber to an outlet fitting during a pressure stroke; wherein the spring is configured in the liquid chamber such that the spring stores energy during the pressure stroke and provides the suction stroke, the pump further comprising a slide valve assembly being movable between a pressure stroke state and a suction stroke state for alternately providing pressurized gas to the second gas chamber such that the liquid passes from the liquid chamber to the outlet fitting, and providing gas from second gas chamber to atmposphere, such that the spring provides the suction stroke and liquid is drawn from the liquid inlet fitting into the liquid chamber, characterized in that the the slide valve assembly comprises a block or housing assembly and an actuator assembly, the block or housing assembly comprises a slide valve housing (22a1) configured with a cavity; the actuator assembly comprises a slide block (22b1) configured to slide in the cavity of the slide valve housing (22a1), the actuator assembly having a slide spring (22b3) arranged between a lower retainer (22b2) and an upper retainer (22b4), and a yoke (22b5) configured with an opening to contain the actuator combination under compression, and further comprising a piston shaft/actuator slide assembly coupling the yoke (22b5) and the piston/diaphragm assembly causing the slide valve assembly to change between the pressure stroke state and the suction stroke state.