it's possible the lines have started to clog up a bit.... it does have a lot of mileage on it. My suggestions would be.....

-Check for leaks at your master cylinder
-drive the car hard, with hard braking and see how much fluid has been retained... if the level has dropped at all... you either have air (doubtful) your lines are leaking
-after you've ruled out both of those and decided that there is no leaks or air in the lines, replace the lines themselves.
-are you sure that your brakes were properly adjusted when they were installed?
-check the level of the pedal and the bolts at the top to ensure it's not just coming loose on it's own.

that's about all i can think of right now... try that out and see where you stand

-Ryan