W124 Mercedes 300D "dieseling" .. won't stop immediately

[Dinoman]

Recently, when I turn off the ignition key, my car will keep running for about 5-15 seconds.
I know there's a vacuum line (brown line) from the firewall to a shut off valve on the engine.
I hooked up a mity-vac gauge to it, and when I turned off the key, the gauge would initially
read only 1/2 a PSI, but, then after a few seconds it would pop up to 5 or more PSI and shut
down the engine. I'm assuming there's a leak in the brown line, or could it be a leak in the
shutoff valve itself? I haven't pulled off the dashboard to take a look at the ignition key yet.
I did have it off recently to fix my cruise control. I could have pulled on the vacuum line then.
Would it be easy to see? Does the vacuum line plug directly into the ignition key module?
Any other diagnostic tips? I'm thinking of pulling off the dashboard and hooking a separate
vacuum line from the ignition key to the shut off valve.
Thanks, Richard