So I'm having a slight "stumble" during a cold idle after I replaced my glow plugs. I've noticed small air bubbles traveling through the lines at idle. After the car sits for a while I have large bubble in the line to the pre filter right where it makes the connection to the top of the housing. I know this is a very common issue with the plastic lines, but I have zero external fuel leakage. How is that I'm introducing air into my fuel without any leakage either during operation or at rest? Is the pump drawing air in through a bad line/seal? I've searched the forum but all related issues are accompanied by fuel leaks.

Picture of the monster bubble