The brass block, or Tee (as it is called in the Parts Manual) is fitted with valves as installed in the A-body. In addition, it contains the switch the activates the BRAKE warning light on the dash.
From looking at the parts manual, the rear wheel cylinder size is a function of the diameter of the drum. The 9" brake used one size piston, 10" & 11" brakes use another. There is no difference whether the car had disk or drum front brakes. Also, there is no difference in the front caliper piston size as relates to the drum size fitted at the rear.
