If the pedal pressure slowy decreases with continuous foot pressure on it, and you do not experience any fluid loss, I would suspect the master cylinder seals.

Because brake fluid is extremely hydroscopic, it tends to deposit the water accumulations in low areas, like the bottom of the master cylinder bore. This inturn leads to a corrosive process that will pit the bore and distort the cylinder's seal.

You could get a rebuild kit and hone out the cylinder, however, without knowing the extent of the corrosion, you are most likely better off to just get a rebuilt master cylinder. REMEMBER, purge your brake system of the old fluid when changing the master cylinder or you may experience the same problem within six to twelve months. Since you have the bleed the system anyways, why not just purge the old fluid while your at it.