I have a 93 Thunderbird V6 w/power brakes. Every once in awhile when you hit the brakes the pedal is stiff and the rpm's run up (naturally a bad combination)[]. My experience tells me either the vacuum check valve or vacuum booster is bad but upon doing various vacuum check, everything is fine.
Am I missing something?

Thanx in Advance
Jim

 

Maybe the valve is intermittantly sticking. With power brakes I've also seen fluid vacuum out of the master cylinder and into the engine through the booster. Booster gets fluid inside. Also air in the brake lines can cause some trouble. Have you checked the fluid level and color? the master cylinder should be full and the color clear to medium yellow (brown indicates trouble with the system).

 

Can you hear a vacuum leak by the brake pedel when that happens. If you do it's most likely the booster. The engine run up and the hard pedel sounds like a vacuum leak and it can happen inside the booster, although you can usally hear it inside the car by the brake pedel.