A pump piston movable with ball-driver device.

The invention relates to a piston pump (1) movable in a casing (2) defining with a first end surface (3) of the piston (1) an inlet chamber (6) of a variable volume fluid according the position of the piston (1), said chamber (6) having an intake conduit (7) of the fluid opening in a wall of the chamber and having a ball valve (9) provided with a seat (8) arranged to opening of the conduit (7), that the ball (9) of the valve is capable of closing in a first closed position in contact with the seat (8, said ball (9) being movable away from said seat (8) in a second position opening of said conduit (7) under the effect of a vacuum created in the intake chamber (6) away from the piston (1). the pump is characterized in that it comprises means (10, 10 ') to unseat the ball (9) from its closing position on its seat (8) in the absence of depression.