Master Cylinder for Disk/Disk Setup

[RichardJ]

Yes, I understood and mentioned that that you had stated your old MC was bad.

The reservoir size has nothing to do with the operation of the disc brake caliper.
Under normal operation, when you press on the brake pedal, there is very little fluid transfer from the MC to the caliper. Unlike drum brakes that have springs to pull the shoes back, away from the drum, caliper pistons and pads stay very close to the rotor.
Only over the course of thousands of miles and the life of the disc pads does the fluid slowly transfer from the MC to the caliper, behind the piston.

Shoe wear adjustment for drums as accomplished with springs and lever turning the star wheel. Pad wear adjustment for disc brakes is accomplished by slowly and permanently transferring fluid volume to the reservoir behind the caliper piston.

The reservoir is sized so that a couple of years down the road, you don't run out of fluid in the MC, before you wear the pads out.
If you never check the MC brake fluid level, then that could be important.

If you must choose a different MC, then choose one with the same bore size as the one you have now and the front brakes which do 70% of the braking will continue to function properly. Checking the booster pin length adjustment is always a good idea when replacing the MC, but imperative when choosing a nonstock MC.