I think I misunderstood your second question. I thought you were indicating the rears were disk brakes. If the front are disk and the rears are drum then the front outlet (to the disk brakes) does not
have a check valve in the outlet and the rear outlet (to the drums) does
have a check valve. The check valves provide a residual 8-10 pounds of pressure on the line to hold the wheel cylinder internal cups against the wheel cylinder wall (to seal it around the cups perimeter). Sorry if I misunderstood the question to start with and confused you.