Ford E-150 Fuel Tank Selector Valve.
Selector valve assy, p/n F1UZ-9B263-B $ 77.99 plus shipping from for my 89 E-150

The fuel tank selector valve assembly.  left side is in from the tanks, right side is to the engine


at the bottom is the o-ring I found in the cup (on the bottom of the valve)

Take off the top and you find this rubber diaphragm.
The diaphragm acts upon this plate in either an up or down force
Below the plate you find these two valves.  These valves direct fuel to/from the two tanks depending on if they are in the up or down position.  One valve is partly disassembled.
The valves are attached to the plate. When the plate moves up or down the valves open different holes in the valve body.
Bottom view of the valve.  the big hole at the upper right is where the fuel enters the cup at the bottom of the valve.


the one at the lower right is the return to the tank valve.  the o-ring I found fits up in this valve

when the rear tank is on, it forces fuel (thru the hole located in the center of the photo) to the top side of the diaphragm.  this pushes the diaphragm down, opening the valves to the fuel lines from the rear tank. that dumps fuel to the cup which feeds the high pressure pump and the engine, and allows fuel to return to that tank.


the the front tank is on, it forces fuel thru the hole in the lower right of the image, which is below the diaphragm.  that forces the diaphragm up, changing the valve so it opens up the front tank's feed and return lines.