Thrust piston pump for active substance dispenser

A piston pump for media dispensers for repeatably discharging a medium such as pharmaceutical products and the like, said pump comprising:

• a pump piston guided in a pump cylinder from a starting position along a pump stroke to an advanced end position and along a return stroke back to the starting position;
• a pump chamber associated with the pump piston, said pump chamber on one hand being connected by an inlet to a suction duct provided for sucking the medium from a medium reservoir and on the other hand being connected to a discharge duct, the pump chamber being defined by a cup-shaped chamber body;
• a second piston, provided to move together with the pump piston, and being constructed as a presuction piston for filling the pump chamber;
• and,
• a suction chamber associated with the second piston, the suction chamber being connected to a duct leading to the medium reservoir and being open over the return stroke to said duct, wherein the chamber body is arranged to define a wall clearance with the suction chamber, the suction chamber having a shoulder-like inner surface, inner surface of the suction chamber by an annular flange projecting over a circumference of the chamber body and having at least one passage opening, and wherein one of said ducts is closable by a valve responsive to differential pressure, means being provided for on one hand keeping the inlet of the pump chamber only open towards the suction duct during a short suction portion of the return stroke of the pump piston adjacent the starting position, and for on the other hand over a main part of the pump stroke keeping the discharge duct closed.