The FSM warns about using the sight gauge to fill refrigerant...
there is a possibility of it indicating need for more refrigerant when it is already full... few things worse for an AC than having too much refrigerant since CHANGING from one phase to another is the basis of the physics... so a proper differential in pressure needs to exist... that is taken away by too much refrigerant...
Fill by pressure and vent temperatures...or .. if a new fill... but accurate weighing of the refrigerant.

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

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

do you know what pressures are needed for the high and low

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http://i166.photobucket.com/albums/u116/OC03/240d.jpg
Current Car:
1985 300DT 55K miles