I have the same problem, I sprayed and oiled everything around my brake pedal to no avail. Then one day I had a friend push on it while I was under the hood and wouldn't you know it, it was actually in the engine bay behind a circular box, ??Master Cylinder?? Everytime he would push on the brake it would push this box out causing it to squeek. I havn't figured out how to get behind it and take care of the squeek, but overtime the squeek goes away and then comes back. Basically its in the engine bay on the oposite side of the brake pedel plunger.
Any ideas how to secure the box so it doesn't move, or access behind it to actually take care of the squeeking part?