I've got a '90 C4 that I installed 6 piston front calipers and 13X 1 3/8" rotors. I modified a C5 master along with a proportioning valve. This works well with stock J55 rear calipers. I am going to install the 13" front J55's on the rear as soon as the new brackets are delivered to me. I am planning on running a modified Chevy truck master with that set up. I tried it with the current setup and it was way too much master. Needed to push with BOTH feet. Well, ok at least one strong right foot that picked me up out of the seat going into turn 12 at Nelson Ledges. (the one and only hard braking area) Anyway, looking for some input, or ideas.

OK a couple of things going on here, power brake booster? Is the master cyl push rod matched or correct length? Are your "feed lines" and or rubber hoses stock or been replaced? All that can contribute to a hard brake pedal, and or correct vacuum going to the booster.