Did the donor truck have rear wheel ABS? If so, then the existing valve was designed for that to kick in and control rear wheel skid. Allowing the rears to activate first and do more of the work (in a loaded truck situation) so the front brakes didn't burn up. However, it never seemed to work right and the rear shoes ended up glazed over, with faulty wheel cylinders, and burnt up front brakes with the attendant customer complaints of poor braking performance and a soft pedal.
It wasn't a great system, back in the '90s when I fixed rigs like that for a living, we had a lot of problems with full size GM trucks and their rear brakes either not working, or locking up too soon. With mushy pedal feel.
Going to a straight bore master cylinder will likely fix a lot of the problems, as you are returning the system to a state where it's designed to work without rear wheel ABS. You're also running the proportioning valve from an S10 which has puny rear brakes and essentially the brake balance of a car, while using a burly 14 bolt rear axle and its much larger brakes.
I would suggest the straight bore master cylinder and finding a prop valve from a 1 ton '90s truck, as long as the donor rig has rear drums. Then the valve will be in a system much closer to what it was designed for.
