I pulled the tank and found the seal broken and brittle.
The seal (O-ring) is located on top of the neck (not wedged inside like the radiator plug)

When I unscrewed mine to repair the float the seal stayed inside the sender put Dow Corning on it and hand tight it back - no problem. Thats DC111 silicone non setting sealant

O-ring dimensions: ID 25, OD 32 mm (3,5 mm thick).