The level shown by your gauge is based on the resistance it see's in the circuit. With the gauge working properly a full tank has the float all the way to the top & the rheostat on the sending unit providing full resistance ~30 ohms. An empty tank would have the float at the bottom where the resistance would be ~0. Knowing that, a gauge that always shows full means the circuit is broken, full resistance. It could be a bad ground as noted by Steve or it could be a broken hot wire (as my '38 had). If you have a good ground and a good hot wire you'll need to look at the sending unit and/or gauge. You can check the sending unit with a multimeter.

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