The fuel sending unit float is made from two pieces of copper soldered together.

Solder breaks down, gas seeps in, float settles to the bottom of the tank.

COAZ-9202-B .. Fuel Sending Unit Float / Available from Ford.

Fits ALL: 1957/79 Passenger Cars/Trucks, 1961/79 Econolines & 1966/79 Bronco's.
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1973/79 F100/350 with dual fuel tanks: There are two different fuel tank selector valves.

1973/74: The valve is manually operated, mounts on the floor adjacent to the drivers side of the seat.

A switch is mounted within the heater control panel. This is only used to switch the dash fuel gauge from one tank to the other.
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1975/79's use an electric fuel tank selector valve and solenoid assy. This valve is mounted on the left (drivers) side frame rail.

The switch is located within the heater control panel, changes the fuel tanks and the gauges.