It's out of equilibrium with the ambient pressure when it pisses fuel out of breather hose. I'm guessing something is wrong with the breather pipe's end that goes inside the tank (it's maybe loose and gets too deep inside the fuel - breather hose should be on top of the tank where no fuel gets) or is it the TT tank that is messing pressure up through the pump system you have for auxiliary tank? It shouldn't be coming out of breather if the tank is in the equilibrium with the surrounding environment's pressure (that's why there's the breather hose by design - there should be no vacuum nor overpressure). I'd look for the details that may create overpressure - i.e. a clogged breather hose on one of the tanks?
Not sure how this TT tank system works tho. Maybe one of the tanks has no breather so the fuel can be pushed (with intentional overpressure) from one to another tank(?) This can be your source of problems.
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