There is a checkvalve in the line going to the brake booster. It's one of the yellow ones and has 4 lines going into it. I knocked the center line loose once durin an oil change and had no power brakes and it wouldn.t shut off. I'd say the valve is either bad or one of the lines are loose.
Hey, Flanagan has a picture posted here
mystery vac line
That shows the valve. You can see it best in the second picture just in front of the filter he's holding.