I can guarantee you that you can have a good vacuum pump and oil under your dash, when your IP shutoff valve dies. Ask me how I know.
You can put a mityvac on that line going to it and find out easy enough.
Those things are $40, read the warnings about run-away engines prior to doing the job so you do it right.
I did a whole thread about this, lots of info here.
My guess is the shutoff valve is toast.

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Think Alternative Energy!
300CD '80 (now gone but not forgotten...)