Brake problem

If you are not losing brake fluid, the master cylinder is about the only suspect. Besides just failing on its own, this happens real often when the driver is surprised by a sudden leak, or when do-it-yourselfers pedal bleed with a helper. Any time the brake pedal is pushed over half way to the floor, the rubber lip seals in the master cylinder are run over the crud and corrosion that build up in the lower halves of the bores. That can rip them and cause a low or slowly-sinking pedal. Often that damage does not show up until two or three days later.

I should have included, if the car has anti-lock brakes, and the master cylinder was allowed to run empty, it may be necessary to bleed the hydraulic controller. Typically a scanner is needed to activate some of the valves so the trapped air can be expelled.