so the rear brake locked up due to the brake fluid boiling over. the caliper was smoking it was so hot.any ideas on what could cause this.im going to get a new seal and piston. since im pretty sure i need to.any help on this issue.i was stopping on the highway due to traffic and managed to hit a side road before it lock all the way up to work on it.i had to crack the line open to releave the pressure so it would suck the piston back in.

 

The piston must have seized and been stuck on the disc causing it to heat up and in turn heat the fluid. It must have been really old fluid as i believe its boiling point lowers as it absorbs moisture.
When was the last time you serviced your brakes?
I would be checking the front brake before riding again.

 

i just worked on the rear a couple of weeks ago. it was doing the same thing but grabbing and not total locking up. i pulled the caliper off and cleaned everything and bleed the system. i rebuilt it a year ago with a new seal and piston and pads and has been fine till now.