Your problem sounds like a fluid return issue. When the brakes begin to heat up, the brake fluid expands. The excess fluid must return back to the master cylinder reservoir through its compensating ports. If the fluid can't return, it forces the caliper pistons out locking up the brakes. If the master is adjusted wrong, the master pistons cover up the ports preventing fluid return. I would also check for any pinched or kinked hard lines. Just some thoughts.