If it doesn't matter which bleeder valve you open, I would suspect the compensating port in the master cylinder is plugged. The compensating port is a small hole, about .020" at the bottom of a counter bored well just ahead of the larger fluid port that is the most visible port in the fluid reservoir. A fault in one of the flex lines would mainly affect one front wheel, or the two back wheels. The back wheels have a common flex line. Compensating ports tend to be ignored during brake work, because they are not very noticable.
Pedal adjustment is also a possibility, and if I remember correctly, there is a clevis that is adjustable on the push rod on the master cylinder. With insufficient piston travel upon release of the brakes, the compensating port will remain covered by the piston and the residual pressure check valve on the MC piston will retain pressure in the system. Add a little heat and you will have locked brakes that need to be released by opening a bleeder valve.
A little more on the Huck brakes: the anchor is at the bottom, and they are not "self-energizing." In the Bendix brake setup, the wheel cylinder is the anchor and the shoes are free to move slightly with drum rotation when the brakes are applied. That movement gives a "self-energizing" effect which improves braking effort without increasing pedal force. Some manufacturers, GM among them, referred to those brakes as "duo-servo" brakes.
Harvester
