I had the same problem on the British made Albion box on the Ebay Bullet in exactly the same place. It was caused by the screw hole being tapped too deep and penetrating the inner case, probably during manufacture. I plugged it with Araldite and fixed it. You could use a fine probe, a wire say, to test if the hole is too deep. It only needs to be pin-holed in one place to leak quite a lot!
REgards, Mark
