Awhile back when I was trying to make a manual 4 wheel disc setup work in my car, I tried a 15/16" M/C from Dr Diff. It didn't help. Nothing did and I tried several combinations of parts. I had different pads, front calipers, DRUM brake combination valves, DISC/DRUM proportioning valves, different master cylinders of various bore sizes...I took a humiliating defeat and put the power brakes stuff back on with a 4 wheel DRUM combination block and now it all works great.
This stuff can make you pull your hair out when the combination is out of whack. It really helps to have an engineered "system" instead of trying to save money by picking and choosing individual parts (AND hoping that they play well together.)
