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. <EOS>