Abstract: The reciprocable plunger of the pump has a valve sleeve which circumscribes the plunger and shifts relative to the latter between port opening and port closing positions in response to telescopic depression and extension of the plunger within its receiving barrel. That portion of the plunger circumscribed by the sleeve is necked down and provided with the port that communicates the discharge passage of the plunger with liquid in the barrel. The sleeve is generally hyperbolic in longitudinal cross-section, having an inner surface which is generally complimentally received within the neck of the plunger. A pair of radially outwardly extending, annular steps in such inner surface on opposite sides of the port define a pair of corresponding shoulders on the inner surface that are disposed to engage oppositely tapering sections of the plunger neck during movement of the sleeve between its port opening and port closing positions, thereby to effect such opening and closing of the port.