I was checking the main line that goes from the pump to the booster, it was not giving a steady pressure. The gauge was moving very fast up and down so fast that I was unable to read it. Is that how the vacuum pressure is on a 1980 300D? Or is it supposed to be flowing in a steady pace? another thing, how many pumps are on this model? None of my locks work is there a seperate one for that ? THANKS

It sounds like your vacuum pump check valve has failed, allowing a two-way flow of air. The door locks operate off the engine driven pump.

Where is the check valve at and how does it operate? where is it located in the car?

The one that I am referring to is in the fitting on the pump. The threaded nipple where the vacuum line attaches to the pump. It allows air to flow into the pump, but not the other way. If you remove that fitting, there is a good chance that its condition will be apparent. If it's still in one piece, test it to make sure it allows only a one-way flow of air. (My experience is with the reciprocating pump; yours may be different.)

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1PY1E2LZD1S30SY3JW&year=1980&make=MB&model=300-D-002&category=D&part=Vacuum+Pump+Check+Valve

so the reading should be in a steady flow of vacuum on the gauge instead of a instable reading? Thanks ill look into it. Do I need to remove the 2 hoses attached to the pump to get out the check valve? Or which way do u recommed to check them. Thanks again

MIne did the exact same thing and it was caused buy a large vac leak. If you disconnect and pulg vac lines and the vac starts to read steady you have found a leak.

Yes, you should get a steady vacuum reading. If your pump has two hoses it is not the same as I am familiar with. So what I said regarding the check valve might not be applicable to that pump.
I would start by getting a vacuum reading from the pump itself, with all other vacuum "consumers" eliminated from the equation. If you get a proper reading then, you know that you have a leak somewhere upstream from the pump.