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

It goes without saying that on a disc conversion, you have to change the MC if you have a single MC reservoir.
I'm about to finish up a Scarebird conversion on a 70 Valiant. The PO installed a new MC before I bought it. It's the MC for a 4-drum system with 2 reservoirs, but they're the same size.
What happens if I don't change the MC? Anyone try this?
I'm not trying to be cheap, just curious.
thanks