Delayed Disc Brake Release

Do you have the adjustable push-rod for the master cylinder? Perhaps the locknut has come loose and the rod has slowly lengthened itself to the point where the brake light switch is applying the brakes even with your foot off the pedal?

That's just a thought. I had something similar at one point -- i noticed that the brake lights were staying on and it turned out my adjustable pushrod had the locknut come loose and the rod worked its way to being shorter. I put in some locktite when I adjusted it back to the right length, and then cranked the locknut too.