Sounds like you either have a leak in the brake system someplace, a line, or you have a bad master cylinder (more likely). One of the seals gave up the ghost and how the brake fluid just bypasses it as the cylinder shaft moves forward and rearward.
Check the fluid after the pedal goes down a few times. If the fluid level nevers goes down, replace the master cylinder. The the level does go down, find where the brake fluid is going and replace the component with the bad seal.
Chris