I have run into this problem on a few late model cars. It seems that the brake fluid degrades somehow. When you step on the brake, and as the fluid heats some of it may boil/bubble/or become less viscous than it initialy was when you first applied the brakes. If you run out of options, evacuate all of your old fluid. You will have to bleed the brakes once the new fluid is installed. Hope this helps.

Regards,
Dave
jmhenneman@frontier.net