Rene,
I re-phrase the first post from Bonzo:
You have 5 liters in the tank and the gauge indicates 0
You add 15 liters and the gauge indicates 15 liters (or 3/4) but in fact you have 20 liters in the tank (5 +15).
The rod must have been bent upward and the 20 liters electrical contact is already ON with a level of only 15 liters of gas. The float should be traveling another 1/4, but is locked by the top connection in the switch. This means that the float is immersed and not floating from 3/4 to 4/4 fuel level.
Now, when the fuel level is down to 1/4, the "0 liters" electrical contact is ON preventing the float to lower itself any further. From a fuel level of 1/4 to 0 the float is not touching the gas and if floating in the air.
If you bend the rod and float by 5 liters downward and it will indicates the right value.
This is the nice thing about a forum, everybody has its opinion and we all debate about the solution.
The constant error caught my attention otherwise, yes like everybody I would have given advices on the electrical connection.
Bert