Have you been driving the truck? For those of us who don't have residual valves installed, do us a favor and add one set at a time and do some testing for us. Add fronts only and go drive the truck around a day or so before adding the rears. Then when you get some results add the adjustable proportion valve and test it.
I know what my truck did once I added the rear adjustable valve, but I don't have four wheel residuals. If I was a betting man, I would bet you see no difference after adding the four wheel valves. The adjustable will keep the rears from locking up, but your pedal will get harder to push under light stopping since you have to add more pressure to over come the valve. With power brakes you may not notice the added effort, but with manuals you will.
The residual valves on drum systems only keep the wheel cylinder piston pins in contact with the shoes, the shoes will always return to the adjustable stop, they will not fix a poorly adjusted shoe.
I can't see how they do anything to disk caliper, the pads only pull back a few thousands of inch anyway, and 2 psi isn't going to stop or limit that. If you ever tried to push a piston back into a caliper, you know how much effort it takes, 2 psi nothing.
My adjustable valve is under the bed just ahead of the rear axle where I could reach it pretty easy. Since installed 25 years ago, I haven't touched it.
