[Jack21]

Proportioning valve is going to be specific to your front/rear brake setup and car it's used on.

If you're buying a set, follow the manufacturers recommendations. If you're cobbling a set together, put prop valve in, then dial it in when you put the car on the street.

If your front and rear brakes are evenly balanced, the worst that could happen is that you'll open the prop valve all the way. If you have too much - too little, the prop valve is how you adjust it.

Every car is different. The same brakes on a Mustang (front heavy, same size tires front/rear) won't balance the same way on a Cobra (50/50, larger tire on rear) e.g. rear brakes can do more work.

Your master cylinder needs to be the right type for your brake system. Verify application through system vendor if not part of kit vendor.

Mixing & matching brake systems is like mixing and matching engine components (Ford block, Chevy pistons, Mopar rods), it'll work if you're certain it all fits, and someone has tried this before.