99.9% of the time, it's always the O-rings and caps responsible for your air intrusion. It's true, it is a cheesy return line system, BUT it is robust and does WORK fine when it is sealed up properly. The problem with most new kit installs, is that the installer didn't get a cap seated right. It takes experience to know if you've got it on right, and if you've only changed these once or twice in your life, you probably haven't used the Force yet to Jedi your way to a proper seal. I've never seen an olive leak air without leaking fuel. Put a clear line between the filter head and your returns, and you will see the air coming from the injector banks.