I marinated the car for a few days with Sea Foam sucked down the brake booster line, and shut off before it was burned. No change.

The vacuum lines are brittle but seem intact upon inspection. The car holds vacuum well after being shut down.

Problem is worse on cool mornings, less during warm afternoons.

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1990 560SEC
1988 300SE
1978 928 euro S engine, big brakes, and more
1996 Suburban TX shark rig
1988 Suburban Pacific steelhead rig
1992 300 TE (recently totalled )