Just a few of my thoughts... I looked at my gas cap and tank as I have them off the bike and in the house. Mine is a non Cali model. I do have a little nipple that is on the underside rim of the gas cap that presses into the little hole when cap is closed. I have no rubber insert in the hole. There are two little holes in the underside of the cap and your cap too. Is it possible that blockage is there. Also, do the caps actually seal all air from getting into the tank? I think not, and may be wrong. But, if no air in then a vac would form in the tank.. So, if some air gets in from the cap, is it possible to simply cap or get rid of all extra Cali lines and kind of convert the tank. This might be dangerous... but, just thinking. Also, I have never filled my tank with gas to any closer than a inch or so of the neck.