The "hold off" valve guys above found in the rear brake tubing is termed a "proportioning valve". It was a separate piece only in the earliest front disk designs. Later, it became part of the "combination valve", hence the new "combination" term. That block also incorporated a "hold-off valve" on the outlet to the front brakes. Its purpose was to delay actuation of the front disks until the rear shoes had enough volume flow to contact the drums. I think it was basically a spring-loaded poppet.
You can buy new reproduction combo valves for ~$70, but I am with 273 that a new adjustable prop valve is better. You can get some that do the whole combo stuff, plus are adjustable, which you really need for any deviations from the factory design (larger wheels, radial tires, ...). The MC doesn't really enter into the equation. All dual reservoir ones output the same pressure on the front & rear circuits.
You don't require a "residual valve" on MC ports today (since wheel cylinders now have washers acting on the rubber cups). That is only for hot rodders who put the MC down low, below the calipers & wheel cylinders.
