I have a 1980 300D with the older-style single diaphragm vacuum pump. It needed to be rebuilt because of a tear in the rubber diaphragm. I rebuilt it on the engine by just removing the cover, and I did it at night. The two little circle check valves that I also replaced were positioned differently I think, and I put them back in facing the wrong way (again night time). When I tried the car there was no vacuum. Then I had to go back in and take all the stuff out of the way again to get to the check valves to put them in the right direction. Once they were put in the right direction, I got 25" vacuum.

I don't know the exact setup for the piston vacuum pump, but maybe some of those check valves are the same, and perhaps they were put back in facing the wrong direction(s). That could possibly explain vacuum before and absolutely no vacuum after.