http://www.mpbrakes.com/troubleshooting.htm

OK it may not definately be a vacuum issue but if it only occurs when you run the AC I highly expect it.

Check out the FAQ's at this link.
Plus they have a general how to diagnose your problem section.

This site hooked me up with brake parts for my dart (nobody made power brake dart parts) when I had it, they can custom make anything you need--for a price