If it is either of your rear tires locking up...
There is a bolt (really just a cap) on the back and a little allen head (5mm if memory serves me corect) under it. Use that to back the caliper out. It tightes down as you pull the brake over time and mine stripped out about 2 weeks ago and locked up. if you wiggle it out it comes right out and you can look at it there. Both rears that have gone out on mine the teeth goe f-ed up on that little screw. I do not know how it works but it seems to be a tattle tail for me atleast.