It reall sounds to me like there is a pipe somewhere snagging or split.
when i first got my ion i had a problem with it leaking but couldnt locate the leak, it turned out to be a tiny hole that was letting the air out. You couldnt actually hear the leak though.
i know you have done it before and over again but i would really advise going over every connection and piece of hose making sure they are seated firmly and correctly and that they are not catching or getting pinched, also check for any little holes. One of the most common places for a incorrectly seated hose is the one going into the solonoid area.