What does this do?

All drum Mopar masters that I'm aware of in the original time we are concerned with came with installed residual valves

You may not need it/ them

You want them ONLY in the drum, not the disk circuit

Your master already may have them

What can happen without them is that the master can return faster than the springs in the wheel cylinders can act, and this can sometimes cause a slight vacuum, drawing a little air into the system