I got my bike into a muddy singletrack today....and as i said earlier my new XT disc brakes were crap powers ( still needed to break in). After some nice mud baths i hit the rear brake and all of a sudden it was able to lock the rear wheel. not perfectly but it had way more power than the beggining. The rear brake had been sprayed with mud from the front wheel so it must helped it break in. The front still was crap. I decided to dismount and pick a wooden stick and blast it with mud all over. After this it had enormous power ( in comparison with the beginning) but when it cleaned itself gradualy power went down as well. I repeated the process and noticed that after mudding it ,it gained power ....and i believe that after a couple of mud rides it will come to its full potential.