That is not the idea that caused this to be put into automobiles. The idea is that the expansion tank, being connected at the top of the system...where the air will accumulate... and a valve which will allow expansion into the reserve tank when hot...and allow liquid only to be pulled back into the radiator when cooling was the way to expel air from the entire system while providing a backup amount of coolant at the same time.
Air entrained fluids do not carry and exchange heat heat as well as liquid alone.

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html