Vacuum System

If I recall correctly there are up to nine systems driven off of manifold vacuum.

When you have a cruise control installed you may have a vacuum booster fitting in the compressor outlet casting that acts as a venturi to provide vacuum even under boost.

The easeiest (best) diagnosis tool is a hand held vacuum pump with a gauge. (about $30) Disconnect the lines from the vacuum tree on the firewall near the brake master cylinder and pump down each line and watch the gauge. Each line should hold vacuum for 15 seconds at least. If a line doesn't then move down the line and test each component. Try to also test the lines. The fuel pressure regulator vacuum line comes from the upper intake manifold. The cruse control line comes from the turbo outlet venturi.