After I take off and hit the rear brake a few times while riding it starts to lock up. So what I do next is crack open the bleeder valve just a little bit to release the pressure and it unsticks the brakes. I also crack open the banjo bolt hose that leaves the master cylinder and the brakes unlock after I open it to. I rebuilt the Caliper, master cylinder and just replace the hose and its got new brake fluid. My guess is when the brake fluid heats up it makes either the caliper or the master cylinder lock up, because I can push on the brake lever and bunch of times when its cold and it doesn't lock up.