It's called a residual pressure valve (RPV). The idea is that it retains a small amount of pressure in the line so that the brakes actuate more quickly. It is used for drum brake systems only — if it is used with disc brakes, it will cause the pads to continually rub the disks.
The reason your new "drum brake" MC does not have one in the rear line is probably because some slightly newer Mopars have the valve built into the rear wheel cylinders instead (I think this is a truck thing, but I'm not sure). I have no idea if you can safely remove those valves or not. I think you will probably need to source the correct MC for your application.
