Any tricks for finding a leak in fuel line ?

Thanks Ill give that a try , but it is not the original tank and also one more thing: if that piece is not soldered correctly how would that make the pickup tube leak? If the tube is one solid piece and is just held in place by that connection why would the tube itself leak ? Maybe I’m misunderstanding you. Thanks

Just a guess on my part but if the tube was punctured or worn through at that location by movement over the years it would not be readily apparent in that location. You seemed to have covered everything else so this is something I thought might be worth a try.