If your brake light reappears...most likely your having a rotor run-out problem. Warped rotors, even slightly warped, will cause the caliper pistons to pump back and forth and allow air to enter the system. Seems the rear calipers are more prone to this than the fronts. This usually happens during longer hwy speed runs rather than in-town stop and go driving. Replacing or trueing the rotors will take care of this problem.