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)
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Theo
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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
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Herman

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