I've had a few issues with things not working and it looks like most of the items are vacuum controlled. On a whim I decided to check the vacuum, sure enough, i'm at 9 in. Hg. It's steady, no bouncing, but it's below the 15-20 per the manual.



Before I rebuild the pump, are there any common vacuum leak areas I should check? I have a few of the vacuum-hose-to-accessory rubbers I'm replacing (like this) but want to know where else to look. Thanks.

Where are you testing vacuum from? You should be getting 20” + at the main line that goes to the booster.

Assuming it’s low at that point, one last think to test is the vacuum check valve. It’s on the top of the vacuum pump.

Which year, engine and model are you working upon?

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Sugar bear, great question. 1985 300SD. OM617.

I took the reading from a connection on the hardline right before it went into the booster. From looking at a vacuum diagram it would appear I intercepted before the Check Valve.

If you have a large leak upstream, it may affect the reading you are getting. You have not yet determined that your vacuum pump is not working properly. Disconnect all vacuum connections from the main hose and plug them (including the brake booster.) Then attach the vacuum gauge to one point. Assumming no leaks in the main hose and all plugged properly, then you are testing/isolating the vacuum pump.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

You may be thinking of the black check valve right before the booster. There’s another metal check valve right on top of the pump. I once worked on a friend’s car that had low vacuum and that was the culprit. Worth looking into

Was able to spend some time with the gauge today. Looks like it's producing 17 in.Hg.

Started by plugging into the port pictured below. This gave a VERY slow rise to 17 in. but stayed very steady.



I then moved the connection to the front of the valve and also got 17 in. however, it was bouncing quite a bit. But only about 1 in. Hg.

Looks like you're working on a W126 .

You need to isolate the vacuum source from everything else before you know what you're dealing with .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better