Pull the line from the key switch out of the back of the shutoff valve. Try to build a vacuum there with the key in the on position and then in the off position. If you can't build vacuum in the on position then you have a leaking line from the key switch down. It has rubber fittings right at the switch side accessable via the kick panel and contortionism...

if you get vacuum on the on position but not in the off position then you have leaks elsewhere. Try disconnecting and plugging lines under the hood until you find the one that is leaking. Remember that you might have to pump up the vacuum tanks so it may build vacuum very slowly.

All this with the car not running of course.

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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