Look at the calipers and see if there are signs of leakage (GUNK). If the calipers freeze up they can "remain engaged" until a sudden change in pedal pressure, releases them (or even NOT). Unfortunately "rebuild" or "replace" the calipers is your only option. Check it out. This happen to me. Great breaking now.
Here is a link to some A1Cardone rebuilts < $50 bucks each (this is the front driver side, same site for the others)
1997-2003 Pontiac Grand Prix Brake Caliper - A1 Cardone - Front, Driver Side, Friction choice, Phosphate-coated
Hope this helps someone.
