Hello.
Many years ago, at least 20, the tank of my 122S cracked where the filler neck is soldered to the tank. My late father had a huge electric soldering iron, that I now own, which we used to clean up and remove the old solder where the crack was and then to resolder the filler neck to the tank. Before you do the resoldering it is important that area to be soldered is sqeaky clean as well as absolutely bare metal. Also use a good flux, if I remember correctly we used a product called 'Fluxite'. The solder that we used was the normal 60/40 lead/tin alloy that the plumbers used. Up until today I have had no further problems. With what I have told you and tried to explain it might be an idea to try and solder the section that is leaking.
Hope you come right.
Greig.

You're talking about the brass fitting on the bottom with the drain plug?

If so, I had that issue with my 1972 145 (my first volvo). It drove me nuts. The final solution was to drain the tank and mix up some JB weld and smear it around the fitting. That sealed it up for good. It was never a problem again - and I lived on a dusty rural gravel road at the time - lots of potential punishment to the tank

I did something similar when the 122 fuel tank filler neck was leaking back in the spring/summer of last year. Has not been a problem since. No smell of fuel in the trunk.

I've always had excellent results with JB weld over any of that two-part clay junk they sell at the autoparts shops. JB is great stuff. Next best thing to a hot weld.

It's likely the rubber seal that's all crispy and hard. You can buy replacement seals.
--
Dale