Dual-Function Reservoir (DFR)
F1UZ-9B263-B Fuel Flow - Top Tank Ports
When the tank connected to the top ports is selected by the dash switch, pressure from it flows to the top of the diaphragm through the tiny O-ring, pushing the switching valves down. The detent rod holds them in this position. The tops of the switching valves open, allowing fuel from the upper 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 top seat, allowing fuel to flow to the top 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.
