I just read the recall. The cast iron one is the one that can cause brake failure and the brass one is the correct replacement. See copy below.
GENERAL MOTORS HAS DETERMINED THAT CERTAIN 1986 CHEVROLET "B"
STATION WAGONS WITH POWERMASTER BRAKE SYSTEMS WERE BUILT WITH
COMBINATION VALVES AND PRESSURE SWITCHES WHICH MAY BECOME
FAULTY. IF LEFT UNCORRECTED THESE VEHICLES MAY EXPERIENCE A "HARD
BRAKE PEDAL" OR "INCREASED BRAKE PEDAL TRAVEL". THESE CONDITIONS
MAY BE CAUSED BY A CAST IRON COMBINATION VALVE ALLOWING AIR TO
ENTER THE BRAKE SYSTEM OR BY A PRESSURE SWITCH LEAKING BRAKE FLUID.
TO PREVENT EITHER OF THE ABOVE CONDITIONS FROM OCCURING, VEHICLES
WILL BE INSPECTED AND THOSE EQUIPPED WITH THE CAST IRON COMBINATION
VALVE WILL RECEIVE A NEW BRASS COMBINATION VALVE. ALSO, PRESSURE
SWITCHES WILL BE INSPECTED AND REPLACED IF NECESSARY.
Link to recall notice.
Brake Bleeding, Rear Cylinders, and Linelock Install