Which master cylinder bore size to use for power disc Kelsey/Hayes

Just to reiterate, I have power disc brakes, all factory setup for '67. It has never worked right, poor braking. Fronts won't lockup, rears will which makes it dangerous. I suspect either bad master, bad proportioning valve or bad bleeding of the system. In any case I want to change the master cylinder cause it's kind of crusty and looks bad.