The fuel transfer pump is located on the bottom of the 3406B/C pump housing and on the side of the 3304B or 3306B/C pump housing. It is activated by the eccentric on the fuel pump camshaft inside the housing and can deliver up to 51 gallons of fuel per hour at 25 psi.
This is a spring pumping piston type pump where actual fuel pressure on the engine may vary from 20-45 psi depending on engine operating conditions. The pump only supplies what the engine requires, plus the amount returned to tank. (About 9 gallons per hour). It is a single piston, single action pump with three one-way check valves. The check valves are: inlet, pumping and outlet. This drawing shows that pumping and fill occur on the same pump stroke. Here, the tappet is almost completely extended and the return spring has forced the piston to the top of the pump. This upward motion of the piston opens the inlet check valve and fuel enters the inlet cavity (green). The pumping check valve at the top of the piston is closed and the piston pushes fuel into the outlet cavity (red). This pressurized fuel opens the outlet check valve at the outlet port. There is no pressure relief valve in this pump because fuel outlet pressure is controlled by the force of the piston spring.