It's also strange that there is no mentioning of how to adjust this "proportioning" valve in the service manual I have for my 1988.
You don't adjust them, the bias is built in.
I don't think a proportioning valve is needed with ABS systems. From what I have read, on other trucks, the proportioning valve only works during hard braking to prevent the rear wheels from locking up prematurely (allowing more force to the front disk
It's really a "combination" valve,, it combines different functions.
1, It serves as a switch that activates a light to warn of a front or rear brake failure.
2, It has a "hold off" feature that retards pressure to the front brakes until the slack of rear shoes is taken up, and the rears start to work.
3, It limits the rear pressure,, on the trucks, the rear pressure is in the 600psi range for the 1500s and the 2500 Suburbans with the JB7 brakes,, although in the 2500s with over 8600GVW, and the 3500s the range was about 800psi.
The pressures and its operation are not perfect,, it's a
passive mechanical device, it doesn't know if the shoes/drums are worn or if the brakes are not in adjustment, and it certainly does not know, or care if the rears lock up.
The combination valve
does not and can not prevent rear lock up,, that's the job of the ABS.