My "how does it leak down" comment was aimed at Optimus who I quoted earlier. If there's no leak, the vacuum will never leak down.

A healthy vacuum pump will pull 20-22" of vacuum. If you have no significant leaks, it has enough volume to maintain 18-20" of vacuum on the rest of the system. To determine if you have bad enough leaks to cause problems, you need to test the vacuum with everything connected. In your photo above, you're tied into the brake booster with everything else isolated. That only tells you the vac pump is working, not if it has enough capacity to overpower whatever leak you have.

As a rule of thumb, a "good" vacuum pod will hold vacuum for hours or days without any noticeable leak down. an "acceptable" vacuum pod may have a very slow leak over a matter of minutes. If it bleeds down 50% in under a minute, it's junk, it may still run for now, but it's going to fail sooner rather than later. If you have original rubber connectors in the engine bay and elsewhere, replace them first. They're cheap to replace and all of the leakage added together can become significant.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)