bubble in fuel line

[genchowford]

I have a 92 f350 7.3. Had some issues with air in fuel line. Installed a clear fuel line just before the filter head. Truck now runs good with no issues. No bubbles passing by, but after it sits for a while, bubble forms as if it coming back from the filter head. Is that normal or where is that air coming from?
thanks

 

[plywood]

Clear line between the lift pump and the filter head, or between the filter head and #1 injector return cap?

In any case when there is fuel pressure then you should have some fuel leaking out of where ever the air is getting in. You may only see a little wetness though.