I now have a plan of attack for tomorrow. What you are saying makes perfect sense, however, the next issue would be; if there is insufficient volume, why?
Main cabin line (which is tee'd into brake booster line, together with the shutoff line to the ignition switch) shows no loss when I pull a vacuum on it with the mityvac.
Perhaps it will all be clearer in the morning.

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2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)

for the advice. Tee'd the mityinto the line to the shutoff valve and found there was less than 10 in.Hg when the key was turned off. Appears that the shutoff valve is NFG; it will shut the engine down with a couple of quick pumps on the mity. However, "quick" is the operative word. Valve won't hold vacuum and line doesn't carry enough volume to overcome the loss at the valve.

Thanks again,
Jim

__________________
2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)