4 wheel disc master cylinder?

I am installing a Master Power 4 wheel disc kit right now. The kit comes with a stock style 73-76 A body master cyl and proportioning valve. The instructions that come with the kit say to disassemble the prop valve and remove the guts, turning it into a distribution block. Then they recommend adding an adjustable prop valve in line if you experience the rear brakes locking. I will be calling them soon for clarification. Good tech line guy there!