Given your expanded symptoms, it sounds like the actuator for the fresh air damper might be leaking. Not sure if it is one you can test or replace without gutting the dash or not.
Using your hand pump (mighty vac, etc) your system should easily hold well over 15" HG, might even be able to draw down to 20+" if the system is tight.
On a side note, my vacuum gauge (not my might-vac) does have positive pressure values as it doubles as a fuel pressure gauge.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
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