Welcome to the MercedesShop forum. If you use the search feature you will find many threads regarding vacuum issues. Here is one that should help.
Your readings sound low. Even if the pump is working correctly, if it doesn't have 4 torx screws on the face, you should replace it. The old ones fail and if the ball bearings stop the timing chain your engine will be toast.

Also if you post in the diesel forum you will get more responses on diesel related issues.

Chris

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


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