This is operated continuously from a cam on the engine camshaft. It provides a constant supply of fuel at low pressure to the high pressure injector pump, keeping it topped up. Some pumps have internal mesh screens to filter out debris in the fuel which could cause the non-return valves to stay open. This screen should be cleaned whenever a fine filter is changed.
The pump normally has an oscillating diaphragm, worked by a lever from the camshaft. Two non-return valves allow flow into and out of the chamber above the diaphragm. Once the pressure in the injector pump body is established the spring which returns the diaphragm becomes compressed and the displacement of the diaphragm becomes just enough to maintain the pressure.
A hand priming lever duplicates the function of the cam. If the lever won’t move more than a little when you are priming or bleeding the system, turn the engine on the starter motor, or hand crank until the cam allows more lever movement.
Lift pumps are not highly stressed and rarely fail but you may want to carry spare diaphragms and other parts, or even a complete pump.