Did you change your brake pads recently, or mess with the bleeder valve on any of the calipers? If so, did you bleed the system of air bubbles?

If you are positive that your not loosing any fluid, the problem is most likely an internal Master Cylinder leak, or the booster.

When the car is at idle and the brake pedal is pressed several times, does the idle fluctuate as if you have a vacuum leak? If so, check the booster first.