Either you'r guage is defective,or you most likely have a vacuum leak.If you can't find any disconnected or bad hoses.Then either it's a gasket leak or the brake booster.The booster is the least likely though.If it is a gasket on the intake,but it's hard to find which.You can spray a SMALL ammount of WD40 along each gasket with the engine idleing.When the idle speed increases,you;ve found you'r leak.Just please oh please,do not use ether to find the leak,or you could end up needing a new engine instead of a gasket.