These sytems are somewhat temperamental, it doesnt take much of a leak to make things go wacky, might be a small enough leak that with the volume of vacuum in the booster it still gives you a good feel there, but lowering vacuum enough to affect other components, if you disconnected the booster ad everything else then worked, I agree that it looks mighty like its the booster doing it, use your mityvac and check both the system and the booster, seperately.