I was thinking more along the lines of double-checking their height adjustment, and making sure they’re not binding when everything’s together. Also, checking the fitment of the float valve into it’s seat for a proper seal.
Maybe that aftermarket petcock is bad? Closed is closed, no matter the pressure behind it. Otherwise, is there anything you can do to get that vent working properly? Short of keeping the gas cap open all of the time, of course! 😉