I took apart the fuel cap and you are correct it only vents one way. Why?
I disabled the one way venting system so now it vents both ways and it pulls air around the fuel filler "moat" Hopefully that will allow the main tank to vent enough so that it's not forcing it up subtank vent. Now the question is how to make the subtank vent so that water can't get in from rain.
I think I can form a wall with some JB weld and shove some gas foam down there to keep most of the water at bay.
We're not out here to rough it. We're here to smooth it. Things are rough enough in town.