[RichardJ]

The rear bias isn't fixed, it's PROPORTIONAL. That may just be why they call it a proportioning valve.
The pressure point where the proportioning begins is fixed for combination valves(the proportion section) and stand alone proportion valves.
Different valves can have a different start point and they can also have a different slope or rate of proportioning.

In the first chart you can see that up to about 600 psi, the pressure front and rear is the same. Above that point, the greater the pressure is, the greater the difference between the front/rear pressure.

600 psi is relatively light pressure. For manual brakes, 75 lb from your foot, a 6:1 brake pedal arm leverage and a 1" MC bore, produces 573 psi brake fluid pressure.

The second chart shows a change in start point for load-sensing or height-sensing proportioning valves. Note the rate of proportioning stays the same.

Proportioning valves have nothing to do with auto-adjust brake shoe adjusters. All that is required for self-adjusters to work is for the shoes to be pushed out against the drum, when the drum is turning backwards.

Dave, you may have a bad proportioning valve(combo valve), but I would first find out what your front brakes are doing. Find a hard packed dirt road or a hard area with a dusting of sand on it and see what it takes to lock up the front brakes.