While not brembo brakes... We had this problem with an RM 250. As soon as we released the pressure in the caliper, the rear brake was fine for another period of time. It would lock up within the first 30 minutes of woods riding. We replaced everything (caliper, pads, brake line) except for the rear master cylinder (which we rebuilt), problem still existed. We found another (used) master cylinder and replaced it, and the problem was fixed. It's been months since we replaced the master cylinder and there's been zero issues with the brakes.