On my truck with 100% factory brakes I used the dual master kit from Jim Carter truck parts. I don’t remember the bore size but i did not use a prop valve or residual valves. Zero issues. The brakes will lock all 4 wheels up at 70mph which is pretty impressive. Thats the master in stock location under floor.
I also just built a ‘41 ford 1 ton with stock master under floor with no residual valves. Zero issues. Brakes are nowhere near as good as the GMC but it has a firm high pedal with all oem sized bores. Its just a ford I guess hahaha.
