[Vwdiesel] Bubbles in the fuel line

[Ethan Vos]

Thanks to all who responded to my fuel filter question.

I am getting bubbles in the fuel line. I have always heard that this is
a result of a leak in the line somewhere. Is it normal for the return
line to have bubbles?

Where is the most likely spot for the leak to be? I can see most of the
line except where it goes through the sub frame under the passenger
seat.

Is it possible that a restriction in the line could cause bubbles from
the pump trying to pull a vacuum?

Will I cause any damage blowing compressed air through the line from the
fuel filter back to the tank?

Under the tank on the passenger side is what looks like a box with a
round thingy that can be turned. It looks like the fuel lines run in
this. What is under there?

Thanks.

Ethan