Where did my gauge fluid go

[T Scott]

The vent in the sender may have become plugged. If that's the case, any pressure in the fuel tank will push the red fluid out of the gauge. This is a simple manometer, and still used today in many applications but must be kept clean. Any crud in your tank could foul the vent. A vented gas cap could help but then you run the risk of fuel seepage when the tank is full. Also, as a side note, blowing the line out with compressed air could introduce water in the line which would have a negative effect.