Originally Posted by sailah
Not sure how I'd go about this. I want to keep that fuel cap vent open so that pressure can't build in the tank
If pressure is building up in your tank, your fuel cap is defective.
The way the cap works achieves two things that your system is not:
1) Slow and little fuel spillage in the case the bike falls on its side.
2) Reduce fuel evaporation.
For normal operation, the auxiliary tank needs no vent.
After you run it dry, a manual vent valve will suffice to re-fill it by gravity.