Proportioning valve options with disk brake conversion.

mvh said

Undersized rear cylinders would make the rear bias worse. It would take less fluid flow to apply the rear brakes — that is the opposite of what you want. You want the rear brakes to come in SLOWER, not quicker.

An adjustable PV solves the entire problem.

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It’s not about the velocity or volume, it’s about the diameter of the piston in the wheel cylinder because that changes the pressure (just like the adjustable valve). The change in fluid velocity/flow moving the tiny, tiny difference in volume by changing diameters is 100% negligible.

It’s not about changing the speed of the rear brakes actuation, you’re reducing the force exerted by the rear brakes. And that’s the same when you install the adjustable prop valve.