I'm not sure if the 83 is the same as the Gen 2/3's, but if it is, there are three vacuum lines:
1. Goes to Intake
2. Loops around and goes into the solenoid on the other side
3. Goes to the Brake Pedal for the Vacuum Dump Valve
If I had to guess, the one you're looking for goes to the brake. Does the line go into the firewall? If it does, then that's probably it. The valve will be at the base of the brake pedal (inside the dash) covered by a large piece of plastic.