Ford Transit 2002 Owner's Manual page 1442

Fuel Tank Level Sensor
The fuel tank level sensor is magnetically controlled by the float mechanism and is used to
measure LPG fuel quantity.
Fuel Tank One-Way Valve
The fuel tank one-way valve opens when LPG fuel is pressurized into the fuel tank supply
line. When the fuel tank reaches 80% full, the mechanical float closes the one-way valve.
This is to allow expansion of the LPG in various temperatures.
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