Help.

I've pored over my fuel system the past two months in effort to reduce what feels like air in the system after prolonged shut-downs. To date, and from low to high, I've replaced the following:

-Tank strainer (and gasket)
-Line from tank strainer.
-Both rear fuel lines
-Lines to and from primary fuel filter
-Primer pump
-Rebuilt lift pump
-Banjo bolt Orings (2) on secondary fuel filter
-Injector return lines
-Overflow valve spring stretched to 27mm
-Cigar hose

First thing in the morning (every morning), I see this:



Clue #1 Upon engine shutdown, standing above that line -return line from IP- a few tiny bubbles work their way back up to the T (I'm deducing that the leak is not coming from the T).

Clue #2 This occurs at no other clear lines. While the engine is running, all clear lines remain clear -no tiny bubbles.

Clue #3/possible red herring While the engine is running, if I pump the primer pump, the clear line from the lift pump grows turbulent and a stream of tiny bubbles appear. This is only when I'm pumping, and I recall a technical term that might explain this phenomenon... something about a sudden drop of pressure releasing entrained air.

What on earth am I missing?

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1981 240D 4sp manual. Ivory White.