A proportioning valve just modifies the front/rear bias of the fluid flow to prevent rear wheel lockup by directing more fluid toward the front, or slightly delaying the application of the front brakes to allow steering control on slippery surfaces such as ice or snow. The diameter of the master cylinder bore in comparison to the caliper piston(s) is going to affect the pedal stroke needed to move enough fluid to get the rear wheel cylinders and front calipers applied. A too-small MC bore is going to result in a long pedal stroke and/or inadequate braking. You're going to need to do some math homework on the fluid volume to determine whether your collection of possibly mismatched parts is going to be safe and effective. Good luck!
Jerry

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