11 or 12" rear rotors?

[JKnight]

You’re accurately describing the function of a different part. It’s a common terminology issue. On cars with front discs and rear drums a “metering” or “hold off” valve was used to delay the engagement of the front brakes until the rear drums could build pressure. It was shaped like a barrel on old GM stuff. You’ll not need this when going to 4 wheel discs. But you will still need a proportioning valve.