Rear brakes lock up... front disc

Seems to me this is:

1 Either a pressure balance problem

or...........

2 a shoe/ pad / problem

You may get to the point where you need to "rig" a pressure gauge, first in one system, then the other to see what you are actually getting. You may need a different prop valve, or smaller rear cylinders.

But don't discount pad/ shoe WEAR IN as well as pad/ shoe LINING MATERIAL. A good example is the two (87/86) Ferd Ranger 2x4's I've had. BOTH had factory rear shoes, and BOTH had know problems where the rear brakes lock up in cold or wet weather. I always had to leave home carefully, and "drag" the brake for about 1/2 a block as well as make a carefull "first couple" of stops to get the linings dried out and warmed up a little. Sometimes this happened again in the afternoon, but generally it's a "once in the morning" thing.

So you might try different types of lining front/ rear.