What if I don't change my Master Cylinder on a disc conversion?

And to equium;
-Almost right.The residual valves are pressed into the exhaust ports of the M/C.I have never pulled one out. I have never converted a drum/drum M/C.
-IIRC that brass valve doubles as the sealing surface for the specially formed M/C brake line. That would mean, once you get it out, you would have to defeat it, then reinstall it so that the joint wont leak.
-Can anyone confirm that?
-I would NEVER tell you to just drill the R-valve out. You might wreck whats behind it. And you wouldnt know that till you drive it.And if a metal drill-chip were to get into the piston chamber, try to imagine what symptom that could create.Thankfully the M/C comes apart really easily.