after going thru 2 BRAND NEW repro tanks I finally took the last one to a local radiator shop.
they did the same thing as yours:
take it apart and re-solder all the seams. Both sides.
they told me they didn't know how the thing ever stayed together - the repro soldering was so spotty and thin.
Very poor workmanship.
It's the only way I've ever had one of these smooth face tanks stop leaking.