Have had this issue before. Talked it over with my buddy the mechanic and he said, change the rubber hose brake lines.
I asked how I can test them... he said it would take longer to explain it....lol
The gist of the scenario is the rubber in teh lines seperates over time and flexing. The rubber then acts like a one way flap or check valve and doesn't let all the fluid back.
Lines are $12 each (don't forget to use the Bonne Bill discount) roughly and the power bleeder is on the shelf to the right of the radio.
00 Regal GS: All kinds of cool stuff.