I had a similar experience with my mini just before Christmas. Mine too is just over two years old. It was reading -30 on the display, so i contacted my local dealership, and they said that they "recommend" the brake fluid be changed every 2 years. Although the guy didn't make it sound too important. In my case however, i was about to embark on a 3000 mile round trip to Italy, so insisted it be changed for ease of mind. IMO its better to have it done, just in case, as the brakes are probably the most important parts on the car. Also the brake fluid change is part of the TLC package, and so costs nothing...zero! Which was nice!!
Good luck
Paul
Good luck
Paul