Most of the time, the culprit is one or two things:
1. Air in the brake lines - requires a thorough fluid flush
2. The master cylinder seals are worn. This can be due to age/frequency of use or when someone bled the fluid, he had someone press the pedal all the way to the floor, repeatedly. Your master cylinder piston typically travels approximately half of the total distance. Over time, the cylinder can wear up to that point, leaving a "lip" in the cylinder wall. When someone pushed the pedal to the floor to bleed the system, the piston seal can be damaged, resulting in leakage.
Start with #1 to see if that fixes the problem.