I had a similar problem on my EW. Inside the tank (assuming they are assembled in a similar way) there are plates soldered across the inside of the tank with captive nuts which have a lead 'top hat' over the nut to prevent leakage. I can see a number of possibilities. The top hat has been pushed up by using too long a bolt, cracking the lead, or twisted due to gunk in the threads or an incorrect threaded bolt being used. The plate has been lifted for the same reason or corrosion has broached the plate/tank join. If the tank has been lined internally then it is possible that the fuel is getting past the liner due to poor application and is finding its way to the weakest point. Think of flat roof leak where the leak can be anywhere on the roof but finds its way to a weak spot a significant distance away from the faulty area.
Pressure testing OK is a good indicator but can mask leaks where the fuel has found a path behind the liner as the pressure can push the liner against the tank making the result look OK.
The tank mounts may be out of true causing stresses on the tank. The solder used on these tanks is good but seems to be age hardened making it less pliable and more prone to stress and vibration fractures.
Hope that helps