I know, i've never read anywhere that replacing the mastler cylinder was necessary for the rear brakes either. Here a good thread: https://www.honda-acura.net/forums/s...+brake+upgrade
It talks about front brake upgrading, but it does mention replacing the master cylinder as an OPTION, but not necessary to do so. Do some more research. I haven't looked too much into rear disc conversions, so replacing the master cylinder may be necessary. Hope this helps!