Yesterday I posted a thread on my 87 abs,cruise, and ac. I agree that it probably is the vac somewhere. I traced it as far as under the brake booster and all was fine. Probably in the dash. Abs doesn't matter. I want this info because the car is for sale. I have all receipts for work on car and parts.If anyone is interested I can be reached at 229-400-3447 (cell) or 229-246-8156 nights. I can email all pictures of the car. Thanks for the replies I got.

From the pass side rear of intake the vac line runs to a check valve as shown.

http://i171.photobucket.com/albums/u309/vetteoz/Checkvalve-1.jpg

The side port goes to the vac ball under the headlight and is teed off to the cruise control pot on the way.
The rear port on the check valve is connected to the vac line that is part of the wiring harness across firewall. (behind dist ) This goes inside the car to operate the HVAC controls.
If anything is broken/ leaking / missing you won't get vac to operate things.