Lex, clever stuff here, this box we can see the insides. I'm guessing air gets in the bottle in the hole for the tubing itself, just needs to be a little bit really. The slick part is the feeding action by drawing and not getting it in the mouth. Is it a dummy cart or using polyfill? What are you using for that brass fitting, and does it point downwards? Could you put up a pic of the innards of the cart? Not trying to pester, just curious about the details.
This'd be quite nice on a mod that has a tight draw and needs to have the atty backed off a quarter turn or so.
Rip, there are feeders that are fed when upside down, just don't have the tube run down to the bottom of the bottle.
edit: was writing too long