I am wondering what exactally a vacuum pump on m617 engine does and what kind of things could you notice when it it about to give. I appreciate the insight.

creates vacuum for the engine shutoff, door locks, etc since a diesel engine doesn't naturally create a vacuum like a gas engine

when its going to go you will notice oil in your vacuum lines in the engine compartment. Its best to install an inline vacuum line filter closest to the vac pump so its a first alert. my last car had one and it had a new vacuum pump on it when I got it.

Your power brakes work on air pressure provided by the earth's column of air above you. That is typically 14.7 lbs per square inch in all directions. The ' in all directions' is used by the vacuum pump by taking out the pressure on one side of a disc.. typically made of rubber which , when the normal pressure of the atmosphere is taken away... pushes from one side... in this case pressing on a rod connected to your non power master cylinder making it into an assisted master cylinder which we call ' power assisted'...
This can be very powerful even when small in size... a four inch diameter disc has about 12 square inches of surface area... multiply that times 14 lbs per square inch ... and you have a serious amount of pressure.
So vacuum is the lack of atmospheric pressure.

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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