i believe tester has jogged my memory.
there is one proportioning valve for manual brakes, and one other for power assist brakes. but you have changed everything, changed drum to disk, and changed MCs too. you may need to find a couple of chevy proportioning valves to find one that works.
as an alternative, remove the proportioning valve, to see if it changes the way the cylinders. (but put it back when done)
you said rear calipers?? disc on the rear? on that year? or you changed that too? this may have required changing the proportioning valve too.