Positive displacement pump with pivot piston valve

A positive displacement pump comprising a cylinder delimiting a cylindrical pumping chamber having a longitudinal axis and including a feed orifice and a delivery orifice at respective opposite ends, a piston pierced by a pumping orifice and associated with a piston drive rod for causing the piston to slide with reciprocating motion inside the pumping chamber between the feed orifice and the delivery orifice, and a pumping orifice closure device for closing the pumping orifice, wherein the pumping orifice closure device includes a closure member mounted to pivot about an axis which is adjacent to the pumping orifice and extends in a direction parallel to said longitudinal axis of said cylindrical pumping chamber, said closure member including a closure face that is substantially plane and that faces the piston, said pumping orifice closure device also including a drive member connected to the closure member, extending outside the cylinder to enable the closure member to be pivoted between a position in which the closure face of the closure member is pressed against the pumping orifice, overlying it in sealed manner, and a position in which the closure face is disengaged from the pumping orifice.