4 wheel disc master cylinder?

In days gone by there was a residual pressure valve in drum brake applications. Its purpose was to hold a small amount of pressure in the system in order to keep the wheel cylinder rubber cups expanded and sealed. Wheel cyinders of today no longer need this as they have different cups, steeel back up plates and / or different shaped internal springs which hold the rubbers expanded. New master cylinders and most reman cylinders, in my experience, no longer have the valve.

I too wondered about the bore diameter for discs versus drum. The master cylinders mentioned above are engineered for disc / drum applications and have the same bore diameter throughout. Works just fine i a disc / disc application.