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The shut off system is quite 'simple'. I spent a long time to figure out my new W124 toy as the PO butchered up the vacuum hoses.
1) The brown hose going into the cabin's ignition. It should always has vacuum as it is tee-ed from the brake booster. It has residual vacuum even when the car is off.
2) The brown hose returning from the ignition to the shutoff valve varies with ignition sw position.
a) Key off or 0 - vacuum. It is fed by the incoming hose thru the shut off valve at the ignition sw.
b) Key is in starting position. No vacuum. vacuum is vent to release IP shut-off.
c) car running - No vacuum.
d) All this is controlled by the mechanical shut off valve attached to the ignition sw.
Hope this help.
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