NW_Pilot's Trans-Atlantic Flight -- All the scary details...

Typically wing tanks are filled to the brim of the filler neck.
Presumably that leaves some air trapped in the tank. Without knowing
the exact placement of the fuel vent pipe intake within the tank, it
is difficult to confirm an over pressure condition in this case.
Absent knowledge of how Mr. Rhine came to his "over pressurizing"
conclusion, it is difficult to substantiate it as fact. Might not the
venting fuel have been merely excess fuel draining from the tank as it
was designed to do when the tank is over filled? After all,
presumably it is the same fuel pump operating in both the factory
designed fuel system and the aux fuel system.

The Aux system used it's own fuel pump and it was tied in after the aircraft
fuel shut off valve.