to recap,
1. you press the brakes, the rears lock up, (front must be stuck on as well but, smaller shoes and don't do near the braking the rears do on a big bolt, your 6400 is a 2-ton BTW, not 1 1/2 ton).
2. You can bleed pressure off at either front or rear and the rear brakes release.
Do you have the proper free-play at the brake pedal (so it is not holding the master cyl down at all? I think you want 1/2-3/4" of pedal free-play if I recall off the top of my head).
If you loosen the line coming out of the master cylinder, does that release the brakes as well?
