I can answer this.

Discs use 2 psi residual pressure.
Drums use 10psi residual pressure.

Now this is why there are drum master cylinders and disc master cylinders. Or more commonly a mix of disc/drum master cylinders.

You can buy the expensive wilwood in-line residual pressure valves, and remove the ones in the master cylinder, or you can use a factory one that's appropriate for your disc/drums disc/disc configuration.

So when you do a 120Y brake upgrade from a 4 wheel drum setup, you should use the 120y master cylinder (Preferably from the earlier non-boosted 120Ys).