The theory is if you go down a long mountain the brakes will be very hot when you are getting towards the bottom which means the performance is not as great. So

The theory is if you go down a long mountain the brakes will be very hot when you are getting towards the bottom which means the performance is not as great. So if you then suddenly need the full power of the brakes it is not available anymore. Also if the mountain is tall enough the brakes might not make it all the way down which has happened a few times in Norway with catastrophic results for large vehicles like busses.