Looks like this is my issue also, best find a restoration guy in NZAs per my external fuel filter post, I have had the tank repaired.
I "consulted" a welding mate who directed me to a restoration company (they do Rolls Royce renos amongst other fine cars) who have a lot of radiator experience.
We determined the leak was from a cracked spot weld on the hinge to tank union. I meticulously stripped paint around the hinge plate, then blew out all the crap and paint stripper with a steam pressure cleaner.
They then sweated solder all around so that it wicked into all crevices under the offending plate. Another advantage is that solder is lowe temperature than welding and will not buckle the tank nor stuff up the visible paint work.
I showed them the POR-15, which they know of and they advised that this would not be necessary after their fix, so i did not do it
It had a few internal spots of surface rust and they gave me a wet product to spray on to remove the rust with a non metallic kitchen pot scourer - looks like new now.
Cost Australian Dollars 340 - Rolls Royce price? Much cheaper than 3,400 or so for a new tank and it is undoubtably fixed "forever", unlike the original construction!
HA! I just posted a thread about this. That is the drain plug. I guess it is supposed to be an AL or copper crush washer. So if its rubber, I would replace it.i have a small leak on this bike i just got, 1999 750ss. the tank previously was sealed, my leak is at the bottom of the tank where there is a small round fitting/vent with an o ring for a seal. maybe my o ring has dried out? can someone explain what this fitting and o ring are for and a fix? thanks!