First day of use was great! But Day 2 has me perplexed. I think pressure in main tank is preventing flow from AdvTank. I'm now in New Mexico. It's 100°. I stop to fuel with 9 miles to empty. Open the main tank, pay at the pump, pump only 1.2 gal and she starts to overflow. I pull the pump and slam the lid. Checked the Advtank and I can see gas. :huh I had the main tank open for several seconds (perhaps longer) before I pumped. I suspect that once opened the AdvTank started to drain in. I pulled the seat and looked at the hoses don't think they were pinched. The main tank was under positive pressure when I opened it to pump fuel. Read through THIS THREAD but still am unable to answer the following: Where is the purge valve? Why is the main tank not venting? Heat? EDIT; So re-read the thread and believe I need a new vent valve with check ball. Or just run a line from the Advtank to ground. EDIT EDIT (kind of like PPS): In the end, I simply removed the check valve all together. Even with a new check valve I was having pressurization issues. Once I removed it no problems. I ran thousands of miles with it this way. I was completely happy at that point.