try this thought----a single master cylinder / not late model duels----

a porportioning valve will cut down the total braking from that one very limited source by reducing the force to the rear

Changing the wheel cylinders is cheap, effective and leaves the total brake effort available---if after altering the rear DOWN by 1/8 inch , he wants more front, he can increase fronts by 1/8 inch--probably cheaper than buying a por valve that has 3 possible leak points