The servo is indeed that big black (or aluminum, depending) thing near the firewall on the right side (just behind the air cooler). It's a nightmare! But, I did succeed in rebuilding mine using some parts from another one or two defective ones someone gave me. It works great now! Ask me if you have any questions about how this works.

My problem has to be a leak, related somehow to the "off" position of the pushbuttons. What I want to figure out is what the vacuum slow is when the switch is off. Is it just a closed valve that might be leaking? Or does it lead to another vacuum line that should be blocked but isn't? Or does it lead to a leaky diaphragm that should be closing but is leaking? If you do have the FSM or diagram for that, it'd be great...thanks

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1980 300SD (daily driver), 1981 300D (Inca Red)