From what your describing, there could be a few things you can check. First, I would check the vacuum line going into the booster, and make sure it isn't cracked and leaking, and that it has good pressure and a tight fit. Then I would check the brake lines. The only other thing that would do what your saying, sounds like air in your lines. You will have to bleed the lines, one at a time, via helms style, to get ALL the air out. That is really the only two things I could think of, that would cause your symptoms. Glazing, on the discs, would kinda do that, but not that severe. If all else, take the discs off, and have them resurfaced, if the problem still persists. Good Luck...