Hold the pedal down very lightly. It should not fade away toward the floor. If the pedal drifts toward the floor and there is no visible sign of fluid leakage anywhere in the system, the master cylinder is leaking internally past the primary cup. Usually, pushing harder on the pedal will result in the brakes working properly as the seal distorts against the wall of the cylinder checking off the internal link.
Check the level of the fluid in both halves of the master cylinder reservoir. Disc brake lining wear will result in a lower level of fluid. This is because the piston moves out in their bores to compensate for the lining wear. If one of the chambers is larger and it is lower in fluid than the other one, the disc linings served by that chamber are probably worn.
If a secondary piston seal is defective, fluid can be pumped from one of the fluid chambers into the other. The result is that one chamber is low, while the other is overflowing.
Look for external leaks from the master cylinder. Leakage can occur past the master cylinder cover when the metal bails are bent and loose. Fluid can also escape past the secondary cup on the rear piston. If there is no power assist, the fluid will be visible on the carpet or on the inside of the firewall or bulkhead. If there is a power booster, fluid will leak from the front of the power booster. A small amount of seepage is acceptable.