Abstract: A reciprocating pump for dispensing fluent substances comprises a piston slidable in a cylinder and defining a substance-trapping space at the inner end of the cylinder. Inlet and outlet ports are formed in the piston and connect with the space under the control of a tubular sleeve mounted on a reduced diameter inner end portion of the piston and capable of limited axial movement relative to the piston. Frictional engagement of the sleeve with the wall of the cylinder holds the sleeve stationary upon initial reciprocation of the piston within the cylinder and the piston slides through the sleeve to switch over the porting connections. During the subsequent portion of the stroke of the piston the sleeve and piston move as a unit.