Abstract: A metering pump wherein the head is provided with internal threads terminating adjacent to internal shoulders at each of the pump cylinder, inlet valve and outlet valve ports. The cylinder, inlet valve and outlet valve each include a flange secured to the shoulder by a collar threadably received in each of the ports. O-rings are provided in a circumferential groove in the body or cylinder of the pump inlet valves and outlet valves downstream of the flange. A piston pump structure includes a bushing mounted to the piston rod between the piston head and a second breakaway stop connected to the piston rod. The piston head includes a plurality of circumferential grooves for receiving respectively a cup seal or an O-ring seal. A suck-back mechanism is provided in a bore in the head and includes a stem having an eccentric cam which interacts with the valve structure to adjust the degree of closure by rotation of the stem.