You only need residual pressure valves if you're switching to disc brakes; return springs move the shoes to the limit of the adjusters on drum brakes. Residual pressure valves on disc brakes prevent the pucks from moving beyond the runout of the discs so they don't have far to move when the brakes are applied.
There is nothing wrong with just keeping the original single master cylinder and maintaining the brake system. I haven't heard of many catastrophic brake failures the had no precursors.
Baldy
