Hi Dave, just want to pass along what I've learned recently with gas tanks.
My only experience (aside form all the other Volvo models that I've owned) with an 1800 is on my 72 ES. I just replaced the tank 2 weeks ago because the pick up pipe in the tank broke at the fitting (inside the tank) as it drops into the tank, and the interior was not worth cleaning. The local Volvo dealer says Volvo does not stock any in-tank parts, yet I've had info through the board that some dealers do. The female threads of the tank fitting are held in place by a round, flat washer- like soldered fitting. It is attached to the double curved pick up pipe with the infamous "sock" which lays on the floor of the tank. I've been told that a propane torch is all that is needed to loosed the flat washer and remove the fitting and pipe from the tank. Obviously one needs a clean dry tank, fume free, or an old school radiator shop. I expect there are p/up /pipes out there in damaged tanks for sale.
I'm just throwing this in because if simply tightening the fitting doesn't work, you need to be careful not to further weaken the joint of fitting and pick up pipe. Mine fell back into the tank weakened by 10 years of non-operation and dried varnish. Eventually got it out through the drain hole. Yours sounds to be in in normal condition. It may be a crack in the male fitting or in the threads. Someone suggested teflon tape on the threads (if you're able to unscrew it). Just replacing the male fitting is another good option. I'dbe interested in your results. Jim Groark