Remove the ground wire from the sending unit to see if the gauge goes past full with the key on.
If it doesn't, this might indicate that the back of your gauge isn't grounded properly. This is common when one gets carried away with painting parts.
If it does go to past full with the key on, then your problem is likely a bad sending unit. Take it out and wire it up correctly so that you can manually move the float up and down while watching the gauge.
Carl


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission