My 1st thing i would do, if you haven't already is take that rear brake master cylinder off. If it hasn't been cleaned or flush in several years, it can clog up the return orifice and will indeed lock the back brake up. That brake line is right by the exhaust and the heat will cause the brake fluid to expand, and if the return orifice is clogged it will apply the back caliper. I've had this happen to me on an 86 I had. A quick way to rectify it is take an 8mm wrench and break the bleeder screw loose to relieve the pressure.