Fuel Senders
I believe the meter will indicate full if the sender wire is open. The sender pulls the indicator to empty; the signal goes to ground through the sender resistor. I you don?t have a good ground at the sender / tank interface it may also read full. If you sealed the sender gasket with pro seal you may have isolated the ground plate. There might be enough proseal under the screws to insolate it. Try using a scrap of wire jumping the sender plate , not the terminal, to bare metal on the wing and see if the indicator drops to empty. If it does all you need is a jumper or putting a star washer under one of the plate screws might work.
Good luck
John