prymertyme
+1y
i'm assuming you are talking about the vent up to the filler neck...it allows air to escape as the tank fills, but since it is behind the cap, it stays sealed when the cap is closed.
best thing i can think of is to either find a way to tee it back into the filler hose, or pump the fuel slowly, so air can escape through the filler neck. it must remain sealed, though, due to the way your fuel system works.