Hi oneoldshoe. I've been waiting for someone to reply but since no one has yet, may I suggest starting with the brake light switch. If there is only a white and an orange wire on it, and the brake lights work properly, that is not the problem. If there are four wires on it, check the circuit that uses the other two wires, (not the orange and white). That style should have continuity between the second pair of wires when the brake pedal is released.
Check the vacuum hose feeding the servo. It typically taps off from the check valve on the power brake booster. Be sure it's not leaking on either end and that is isn't cracked or dry-rotted.
Tug on the throttle cable to see if it is broken or disconnected. If it pulls out on the carburetor end more than an inch, remove the housing on the front of the servo to see if the clip broke that holds it to the diaphragm. If you can pull the cable all the way out, it will be rather obvious it's broken.
From what I can remember, you should be able to measure voltages at the wires that plug into the servo. Use a test light to measure on each wire. What you're looking for is a voltage that you can change. What I mean is with the ignition switch on, you might find 12 volts on one wire that goes away when you press the brake pedal. A different one might have 12 volts when you press the "set" button, etc.
The next thing is to look for the speed sensor. On some models the lower and upper speedometer cables both screw into the servo and the sensor function is a set of mechanical flyweights, not an electronic sensor. If the cables do not screw into the servo, there will be a speed sensor either on the transmission where the cable attaches, in the middle of the two cables where they connect, or there were some that had an optical sensor on the back of the instrument cluster where the cable attaches.
It was somewhat well-known in the '80s that GM had a lot of trouble with their servos and it was common to find replacements for three-year-old cars were no longer available. (I worked at a Sears Auto Center at the time and installed a lot of aftermarket cruise controls to replace the systems on GM cars).
Any chance you could post a photo of the servo and the wires on it? In the meantime I'm trying to find a service manual that shows the wiring diagram.