Don't know much about that part except that the Vacuum is supposed to start of high when your foot is not on the pedal and the closer you get to full throttle the closer to zero the vacuum gets; here are some threads:

http://www.peachparts.com/shopforum/showthread.php?p=869411

http://www.peachparts.com/shopforum/showthread.php?p=834716

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84 300D, 82 Volvo 244Gl Diesel