The shutoff valve has a diaphragm in it that goes bad and though it may still shutoff the motor the vacuum system continues to pull vacuum until the system bleeds off completely and it slowly pulls oil up from the IP to the switch.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique