Low vacuum reading

I have been having some shifting troubles, and I tested the vacuum at the main vacuum line "T" in front of the brake booster. I was told that I should
have 21-23 inches HG, but I only have 16. Does this mean my vacuum pump is bad? I wasn't sure if there was any way to adjust the pump? Thanks for the help!
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On what car/engine???

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Karl wrote:

Sorry, 1983 300SD
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That helps.....
Remove the line on TOP of the vacuum pump. Get a flashlight and look in the fitting on top of the vacuum pump. Do you see a check valve in it? Or is it an open fitting? More than likely, it will be empty. If it is, go to your MB dealer and buy a new fitting. It is available separately. Remove the fan and fan shroud. Remove the screws from the cover on the vacuum pump. Carefully remove the cover and WATCH the position and the direction of the two check valves in the front. Remove the top one. Fish out the 3 pieces of the broken check valve that fell down from the top fitting: a flat piece of metal with 3 ears [that is the piece that broke], a spring, and a dark grey plastic piece. The replaceable fitting/check valve is #20 here: http://www.detali.ru/cat/oem_mb2.asp?TP=1&F 3193&Ma7%2E952&GAr2%2E120315&CT=M&cat3&SID&SGR    0&SGN
Part number 004 997 18 72
Available here also: http://www.importeccatalog.com / Only a buck cheaper than the dealer list price.....

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If you're sure there's no other leaks (I'd test right at the pump) then yeah that's weak.
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