Also, a disc drum type of car requires a prop valve since drum brakes are non-linear and disc brakes are linear. So you need a prop valve to "blow off" the excess pressure to the drum brakes or else they'll always lock up.

A 4 wheel disc car does not need a prop valve if the caliper piston size is correctly matched. The piston ratio front to rear depends on the weight distribution of the car as well as the height of the center of gravity. Basically what you are trying to do is to match the brake force with the weight on the wheels. So a nose heavy car needs more front brake force than a car with a 50/50 weight distribution. And a car that is jacked up in the air transfers more weight to the front during a stop than a car that is super low to the ground.

A good chassis should be around 50/50 weight distribution and low weight transfer to the front and would need 2x the brake force up front than in the rear. So the area of the front caliper pistons needs to be 2x the area of the rear caliper pistons. If you follow that forumula you should be in the ball park. You can tune it with the pad selection. If it is way off then you need to adjust the line pressure or the piston size or the rotor diameter.

It isn't black magic, just regular old engineering calculation. Randomly selecting brake parts out of a catalog won't work any better than randomly selecting pistons and camshafts will work when building an engine.