Disc/disc vs. Disc/drum Masters

[68Formula]

What is the difference between a 4 wheel disc Master and a Front Disc master only?

I read in some places it's not necessary to change over as long as you have an adjustable pressure valve and others say it needs to be done.

I switched from 4-wheel drum to front disc already. I have the Baer Track discs, their master, and a stock 13" booster. I used an adjustable prop valve.

Now I have a rearend that came with LT1 rear discs setup already installed.

Thanks.