Unless you are losing brake fluid or have a rusty brake booster, I would look at the pump needing rebuilding. To check vacuum for this, anywhere along the large line from the vacuum pump to the booster would be a good place to find out what the pump is putting out.

The pump is fairly easy to rebuild. The only thing I ran into is one of the hex drive screws was rounded, (Hint: remove the ones hardest to see and access first.) and the rebuild kit was not the correct one. (Ensure it is for a "late-model" pump). You can replace the check valves, etc. while it is on the car but you will be unable to verify the condition of the hardware side unless you remove it.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )