4 wheel disc - recommended master cylinder

Good evening all,
I recently finished installing a 4 wheel brake kit on my 71 Duster so I could have the 5x4.5" bolt pattern. Fronts are the typical 73' style but rears are the MBM setup.

The car previously had the manual brake Kelsey Hayes front but drums on the rear . As expected the master cylinder does not work well with the new setup. I prefer to keep it manual but am open to suggestions. I also read I should gut the proportioning valve so it acts as a distribution block. Ant thoughts on that too?

Thanks in advance.

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