Brake question....

I went to look at a 70 Dart yesterday and I have a question reguarding the brakes. First the car has been switched to disc brakes from a 74 Duster. The pedal felt fine but you could hear a whistle or howl I should say. I thought it could possibly be a vacuum leak but the noise went away if you just rested your foot on the brake pedal. Would that be a vacuum leak? Anyway to test the booster to see if it is an internal leak??