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

Ditch the P-valve? You better have a good reason for flat out recommending that!
-Disc brakes are not self-energizing, as are the drums. So by design they require more pedal pressure to operate.Thats why the factory db cars usually have vacuum boosters on them.So now,with all this hydraulic pressure available,what do you imagine will happen when you get on the brakes hard? Thats right, the rears will lock up early.And what happens to a vehicle when the rear brakes are locked up? Thats right, they tend to swap ends. So if if you like that kind of thing,then by all means ditch the valve. But I wouldnt be recommending it to others.
-Furthermore,That valve is actually a combination valve.In addition to the proportioning function, it also has ,built in, the brake failure warning switch.On cars with the dash-mounted brake warning lite,(cars with split braking systems)ditching the valve will defeat the purpose of the lite. And no the primary purpose of that lite is not to remind you to disengage the park brake.Its there to tell you one side of your split system has failed, and your car is unsafe to drive.
-On the other hand;If your car has non-factory tire sizes on it, particularly, big and littles,Then it may be possible to ditch the proportioning side of it.
-When I fitted 295s out back, I was able to gut that valve,leaving the switch in it, and gained a goodly amount of rear braking capacity.
-Also; getting air into the front brake lines in a split brake system, is not an immediate catastrophic failure. Remember the rear brakes are still functioning 100%. And the first time air enters the line, you hardly even notice it. After a bit, the pedal seems soft, but people continue to drive.Its not until theres a real bubble in there that people search out a mechanic.I used to hear it all the time.They will say something like,"yeah,the pedal hasnt seemed right for a while now....."

- And to nm9;Not only did i make assumptions, I took liberties. And I completely forgot that those drum/drum MCs were that much smaller.Thanks for the clarification.
-But even at 50 CCs,theres still a lot of wiggle room,for those who regularly check the fluid level. Also I was responding more to the posters who were imagining brake failure on the first application,than to those who never check the fluid levels.