Yes, venting the main tank will work. If I understand what you are suggesting, you are saying to put a fitting in the bottom of the tank and then run something (tubing?) up from that fitting to the top of the tank: yes, that should work fine. If you tie the new main tank vent to the aux. tank, then even if that internal tube should fail in any way, the system won't vent fuel overboard, it simply won't fill very fast and that will alert you to the problem.
Actually a system like that will even help prevent the main tank from ever overfilling, at least very fast. Once the main tank is full to the vent you install, the whole process of transferring fuel will either stop outright or continue but very slowly.
As I think about it, that is a great idea really. Your ideas for internal tubing are good but any long tube attached at only one end is subject to failure as time goes by; it would really help the situation if you could anchor the other end (top) of that tube to the fuel tank somehow. Maybe some type of clip that attaches to the filler neck spout? Like a band wrapped around the filler neck with a protruding piece on the side to hold the end of the tube in place. You can also use a piece of rigid tubing if you mount both ends and done reasonably well, that whole thing should out-last the bike itself I think.
Not having thought about this too much, I would say a second vent line tied to both tanks and a strictly gravity fed aux. tank would be the best way to go. No electrics to fail, no wiring, no additional plumbing in the fuel line that could leak, and no vibration from a pump that also may cause leaks.
Physics is your friend.... sometimes.
And it seems to be so in this case. I would like to have an aux. tank a bit lower on the bike and that just won't work without a pump.
Brian
Brian, I dont plan to continue this way it was just an experiment for fact finding. While riding ,was thinking may something like a valve on the front of the tank to admit air. I kept thinking about something similar to a pressure relief valve or bleed valve on a compressor or a rollover valve in reverse. Just ideas, I really dont know what is available for an application such as this. I run with a tank bag so popping the cap really is not an option. On another forum it was suggested that I install a compression type fitting with a piece of brake line going into the tank down low near the bulkhead fitting up to the opening and plumbing a flexible line from the tank to the vent on the aux tank. What are your thoughts on this. I really do not want a pump on the connie only as a last resort. John