As I ride for an extended period of time my brake lever gets harder and harder to apply. Last week when I was riding the brakes seemed to get so hot that they locked up and the sled wouldnt move. I let them cool down and the brakes worked fine again and didnt have that problem the rest of the day. Rode today and brakes seemed to do same thing, get hot and then hang up. They never locked up today but I could tell they were hanging up because the sled would slow down in a hurry as soon as I let off the throttle. Any idea what could be causing this?