Had the same problem end of last year on my 71 GTX. Whole brake system is fresh, new calipers, master, hose, etc. only thing reused from the assembly line is all the valving. Had about 500 miles on it then all of a sudden the front brakes started locking up. Cool it down and it was fine. Just getting it out of the garage today and see if the brake genie showed up to fix it. All I can think of is the calipers hanging up, time will tell.