A reciprocating piston liquid pump and piston driver mechanism includes a reciprocating piston liquid pump having a piston chamber and a piston reciprocating in the piston chamber from an unactuated position to an actuated position, with movement of the piston from the unactuated position to the actuated position causing the advancement of product. The piston driver mechanism includes a first driver member having an axis and a sloped circumferential surface extending along its axis, and a second driver member having an axis and a sloped circumferential surface extending along its axis. The first and second driver members are aligned along their axes and mate at an unactuated position along at least a portion of their sloped circumferential surfaces. Rotation of one of the driver members about its axis relative to the other of the driver members causes the second driver member to advance away from the first driver member toward an actuated position. This movement also causes movement of the piston to its actuated position, thus advancing product.