A piston pump with an improved arrangement by which a piston-forming element is movable relative to a piston chamber-forming element between locked and unlocked positions. The piston chamber-forming body has a collar member having an inner guide tube coaxially about an axis with a lug member extending radially inwardly therefrom and the piston-forming element has a slide tube coaxially radially inwardly of the collar member with the slide tube carrying motion control features for interaction and engagement with the lug member whereby relative axial and rotational movement of the piston-forming element relative to the piston chamber-forming body provides for the adoption of positions in which the pump is operable to dispense fluid and positions in which the pump is inoperative.