On my 83 300SD the shut off valve vacuum line is brown in color. Here is an easy way to determine if that is the problem of "shut off mechanism". I don't think 85 vacuum digram has that much difference.
if you don't have a vac gauge...
Locate the main vacuum line connected to the brake booster, you should see two vacuum lines(one is brown and the other one is white) connected to a Y connector which attached to the T connector of the main vacuum line. with the engine running turn the key to "off" posistion(or remove the key from IGN) to shut it down. Now, go out there and disconnect the "White" vacuum line and use your finger to plug that open Y end. if the engine shut off then there is a leak somewhere else that the white vac line goes... If the engine doesn't shut off, disconnect the brown one too and put another finger on top of that open end to see if there is any suction...if not then the T connector is clogged.(assume the pump is working since you didn't mention brake is hard). If you feel the suction then locate the other brown vac line on the wall next to the brake booster mounting surface and trace it down...it should be connected to the"shut off mechanism" on the back of the IP. disconnect it from there and connect a spare vacuum line directly to the Y open end, close the end with your finger. it the engine turn off then there is a leak where it connect to the IGN switch. otherwise it's the"shut off mechanism"
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