I believe its only the SD's of those years that had the electric vacuum pump for when the car is off. I've only checked the vacuum right out of the main line but I got 22" at idle. Never checked it anywhere else.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo