As you know drums use RPV and discs do not. Earlier drum setups used an RPV in the MC and later on they switched to using a metal quarter sized disc behind the wheel cyls' neoprene cup seal to do the same thing (better seal/no air in/quick take up on the pedal). Stick a drill bit into the brass cone seat & you'll feel the RPV if it has one (very few MC's do now) as it is just a piece of neoprene and a spring for a crude 1 way valve to maintain a minimal psi. The drum half can get by with or without an RPV but discs cannot have one other than an aftermarket 2 psi one but that's not applicable here. Get a disc/drum MC now that has the larger front disc half & you're set for when you later switch to discs in the front. I believe Eberg (eBay) has rear wheel cyls with the metal discs but the main adv of them is he has em in the smaller bore size.