Originally Posted by JimBlake

If that doesn't work get a new master cylinder. That can leak internally, back into the reservoir. If you don't bleed/flush the fluid regularly, there's crud in there. When you bleed the MC piston gets stroked all the way down where it never normally goes. Abrasive junk cuts up the edge of the piston seals.

Sometimes you can avoid this by using a pressure bleeder or just NOT pumping the pedal all the way down. (Put a piece of 2x4 under the pedal while bleeding.) Sometimes it just happens anyway.