Brake Question

[bafflefinder]

Along these lines, just bought a72 malibu and the rear drums were a mess. I replaced them with discs so discs on front and rear. I also replaced the master cylinder. I bled the master but can't bleed the system. I have 1/2" free pedal before the rod depresses so I am sure I am getting full rod release at the master. I believe I have to replace the proportional valve with a distribution valve since the rear discs don't have a spring to overcome. Is this correct and is this all I have to do?