What Master Cylinder? and more

I did not have to do anything either when I changed mine to
C body brakes. I did the same thing as rumblefish360 and it
works great. Sorry I don't remember which year master cylinder
I bought.

They sell adj. proportioning valves for the propose you are looking
for. I don't remember changing my proportioning valve, IIRC I tested
the brakes to see how it worked and I found no need. For me I have
better brakes than I did with the power brake setup.

Just in case as you may know this. If you have power brakes now
you will need a different (I think shorter) brake rod or whatever
it's called.