Well, i guess I have my weekend plans now.

 

I bet you find a broken braze.

 

If you back pressurize the vent hose the only area where air should be exiting is at the small hole in the tank fill recess. If air is escaping the general fuel tank area this is a clue that theres an issue with that inner vent tube . (At least in theory i think this should be the case) . Keep us informed ! Maybe buy one of these bore scopes to visually see inside the tank. (I have one they are really handy )


https://www.amazon.com/Seesi-Endoscope-Waterproof-Inspection-Semi-Rigid/dp/B07PBF6DX5/ref=mp_s_a_1_3?crid=3TQF0HJP3OFI1&keywords=endosco pe&qid=1683120217&sprefix=endoscope%2Caps%2C169&sr =8-3

 

I don't even have a vent tube on my tank. Removed it never a problem in last 4 years. However I have a two inch tank lift.

 

I have a fatboy s. Had same leak out vent once filled full
i found mine. Was vent tube inside tank was never hooked up to roll over valve
i secured it to valve. Reassembled all parts and now i can fill my tank with no leaks