Diaphragm pump for receiving drive power from a motor, said pump comprising: a housing having a pumping chamber adapted to contain fluid to be pumped, a transfer chamber adapted to contain hydraulic fluid, and a hydraulic fluid reservoir; a diaphragm having a transfer chamber side and a pumping chamber side, said diaphragm being disposed between said pumping chamber and said transfer chamber and adapted for reciprocation toward and away from said pumping chamber; a piston in a cylinder in said housing adapted for reciprocation between a power stroke and a suction stroke, said cylinder forming a portion of said transfer chamber, said piston including an interior space and moving longitudinally in said cylinder with said cylinder having a surface with an upper portion when said pump is oriented so that said cylinder is generally horizontal; a fluid communication path for the hydraulic fluid between said hydraulic fluid reservoir and said transfer chamber and a valve in said path for selectively allowing flow of hydraulic fluid from said hydraulic fluid reservoir to said transfer chamber when said valve is open; characterized in that said valve has: a valve housing inserting into said interior space of said piston, said valve housing including a circumferential groove which is axially located so as to intersect with a valve port of said valve housing; a vent between said transfer chamber and said hydraulic fluid reservoir, wherein said cylinder has a wall as a part of said housing and said vent is a passage through said wall providing fluid communication from the upper portion of the surface of said cylinder to said hydraulic fluid reservoir; wherein air in said transfer chamber is forced from said transfer chamber through said vent in said cylinder. a housing having a pumping chamber adapted to contain fluid to be pumped, a transfer chamber adapted to contain hydraulic fluid, and a hydraulic fluid reservoir; a diaphragm having a transfer chamber side and a pumping chamber side, said diaphragm being disposed between said pumping chamber and said transfer chamber and adapted for reciprocation toward and away from said pumping chamber; a piston in a cylinder in said housing adapted for reciprocation between a power stroke and a suction stroke, said cylinder forming a portion of said transfer chamber, said piston including an interior space and moving longitudinally in said cylinder with said cylinder having a surface with an upper portion when said pump is oriented so that said cylinder is generally horizontal; a fluid communication path for the hydraulic fluid between said hydraulic fluid reservoir and said transfer chamber and a valve in said path for selectively allowing flow of hydraulic fluid from said hydraulic fluid reservoir to said transfer chamber when said valve is open; characterized in that said valve has: a valve housing inserting into said interior space of said piston, said valve housing including a circumferential groove which is axially located so as to intersect with a valve port of said valve housing; a vent between said transfer chamber and said hydraulic fluid reservoir, wherein said cylinder has a wall as a part of said housing and said vent is a passage through said wall providing fluid communication from the upper portion of the surface of said cylinder to said hydraulic fluid reservoir; wherein air in said transfer chamber is forced from said transfer chamber through said vent in said cylinder. a valve housing inserting into said interior space of said piston, said valve housing including a circumferential groove which is axially located so as to intersect with a valve port of said valve housing; a vent between said transfer chamber and said hydraulic fluid reservoir, wherein said cylinder has a wall as a part of said housing and said vent is a passage through said wall providing fluid communication from the upper portion of the surface of said cylinder to said hydraulic fluid reservoir; wherein air in said transfer chamber is forced from said transfer chamber through said vent in said cylinder.