when I converted my manual drum brake 68 Polara over to 73 New Yorker front discs I kept the non power master cylinder- I had collected a couple of power units but hard to get parts for them or rebuild so I never installed - worked
I did put in smaller rear wheel cylinders to prevent premature rear brake lock up
and I checked the check valves in the master cylinder and removed and replaced as necessary
never needed and adjustable proportioning valve
but it would lock the fronts first on ice- that's where a light pedal plus pushing on the parking brake while holding the release up comes in- you can still stear