I ASSume we are talking discs all around, this is the race section after all. If so NO prop valve is needed, the piston size, just as in drums with a cylinder will "self regulate". Do yourself a favor and buy a brake pressure gauge and learn about the relationship of pedal ratio and pressure. If you simply want to be able to adjust the amount of fluid moved between the front and rear then by all means ad a prop valve. But it is not a REQUIREMENT for a 4 wheel disc set up.

FWIW I have NEVER run one with a 4 wheel disc set and never had an issue. This includes cars I have built for myself and other to include cars with carbon and carbon/steel rotors. I've owned two ex pro stockers and they do not use one. All my cars we use a brake pressure gage to set them up and with pedal pivot/arm relationships and m/c bore size to achieve the correct pressure its never been an issue.

If you have front disc and rear drums then yes you will need a prop valve. The reasoning is the area of the wheel cylinder and disc piston being different.