Can you replicate the no shut-down in your driveway. If so you can troubleshoot the vac system from there. Could be the pump I suppose, but may just be leak somewhere else (although, it sounds like you covered some ground there). Make sure you don't have any oil in the vac lines that might be screwing with things. You might pull the instrument cluster and test the shut off valve at the ignition - but if you are having brake problems too, it probably isn't that. Hmmmm.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman