I've been running disk brakes ( '70 - '79 Camaro/Firebird ) since 2000 with out residual valves. I have a combination valve from a early '80 s Chevy car, and a adjustable proportioning valve on the rear brake lines. The rear adjustable is pretty much mandatory due to the lack of weight on the rear axle during braking.
I have never seen the need for the residual valves, why do you want the brakes to drag? The reason I always hear is, they keep the fluid in the line and not overfilling the master cylinder since it's frame mounted. So how does the fluid return, you ever try to push a caliper back in? Gravity in a little brake line nots going to do it. How did the truck stop for the first 50 years with the valves in the lines?
A simple test, go out and step on the brake pedal like you normally due to stop the truck, measure how far down it goes.
do it a couple ties with 5 to 10 minutes between each test.
Remove the valves and bleed out the system.
Repeat the above test and see if the calipers are retracting farther. Wait and hour so the fluid has plenty of time to retract and measure the pedal distant again.
