I had vacuum problems and when I rebuilt the pump I had absoloutely no vacuum as you did.... I just figured it was due to never doing anything mechanical two months prior (the white car was a tremendous learning experience
I ended up just buying the pump like you did, worked like a charm!
When you are going through the fun ordeal of changing the brake booster also make sure to flush all your brake lines with fresh brake fluid. There are many reasons to do this, and it is also mentioned in the MB manual as an annual preventitive maintenance item. That and it just makes sense to change fluid that cames out of the bottle clear, and has turned black in your resevoir and lines!!
Best of luck in your project
John
'79 300SD