[DavidG]

They are know as hydrostatic gauges and were used on all Fords from '32 through the '35 models. The more fuel in the tank, the more air pressure in the air and therefore a higher level of red fluid in the gauge on the instrument panel. Those little cups on the tank sending unit help maintain the pressure to avoid gauge fluctuations when cornering or going up or down hills on less than a full tank of fuel. The air line tubing on the back of the gauge on the instrument panel is very small in diameter and if it is blocked, the gauge ceases to function. If there is an air leak at the fittings on the tank unit or the gauge or in the air line itself, obviously the gauge won't function as designed.