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This is what a proportion valves does. It allows for the same pressure front and rear up to about 500-600 psi. After that, the rear gets increasingly less pressure than the front brakes. Too much pressure to rear drum brakes on a Disc/Drum setup will lock the rear brakes up.
Very few production vehicles from that era have stand alone proportion valves and they are non-adjustable. These were used on 1st generation Camaro, Chevelles and the same one was used on B body Mopars.
Drum/Drum did not use a proportion valve, just a block for the switch.
The combination valves that I have in my other post were more typical and is what were used on these trucks.
Combination valve has three parts.
1. Metering valve - or hold-off valve - prevents any fluid from flowing to the front discs until the pressure reaches 40 psi. At 39 psi there is no pressure at the disc brakes, but the 39 psi on the rear drums will cause them to start slowing the car. The metering valve provents nose-dive when you start applying the brakes.
2. Brake switch - indicates brake failure. Same thing that you have on the truck now that was used on Drum/Drum brakes.
3. Proportion valve - As described above.