If you perform the test described in the previous post and find that the shut off valve is good, You can test the switch by just putting your mighty vac in the same "circuit" but before the switch. I would guess that your problem is due to a bad shut off valve. I had to replace the original in my '78 just last December. its a bummer because ita take like 30 min to change it out but the part is gonna set ya back 'bout $100-$178. A bad Ignition Switch would probably be worse.

I would also insure for proper function of THE vacuum pump while you're at it. Its not impossible that you could have an "over all" vacuum problem. do you have problems with any of the other vacuum related functions with your car?

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"