Dual-Function Reservoir (DFR)
F1UZ-9B263-B Fuel Flow - Bottom Tank Ports
When the tank connected to the bottom ports is selected by the dash switch, pressure from it flows to the bottom of the diaphragm through a journal in the body, pushing the switching valves up. The detent rod holds them in this position. The bottoms of the switching valves open, allowing fuel from the lower tank-side supply (large) port to flow to the supply check valve which opens, filling the reservoir. As it fills, fuel is forced up the pickup tube & out the engine-side supply port.
(Note the lower diagram shows the opposite side of the reservoir.)
All fuel returning from the engine flows to the return check valve which allows any excess into the reservoir. Because both switching valves are connected to the lower diaphragm plate, the return valve also opens its bottom seat, allowing fuel to flow to the bottom return port & back to that tank.
A leak at the switching valves' O-rings can allow fuel from one tank to return to both tanks, giving the appearance of high fuel consumption (low MPG), and possibly overflowing one tank.
