If the rod between the master cylinder is too long, the brakes will lock up just as roofcam described. But, normally when the rod between the master cylinder and booster is too long the brakes will remain locked up and switching the engine off will have no effect. I've had to adjust that rod on many cars to fix this very problem but I've never seen vacuum make a difference. If the rod between the pedal and the booster is too long and it is applying pressure to the booster, it could cause the pedal to move down a little and lock up the brakes. The reason I didn't mention rod adjustment in my first reply is it sounded to me like you meant the vacuum was pulling down on the pedal as if a ghost were sitting in the seat pressing the brakes...I might have been too sleepy last night wheI read it though. Can you actually see the pedal move down when you start it up? I would definately check those adjustments as the others said but if the booster pulls the pedal down by itself when vacuum is applied, and those rods are adjusted properly, you've got a bad booster.