Brake line tube diameter question

Could also be the location you live. Those cars were made in multiple factories in the U.S. and Canada. Parts for them made in many locations. All the valves i would see here including the ones on the full size pickups and S10s that had the prop valve mounted on a bracket next to the master cylinder were brass. Only time i would come across cast iron ones was early to mid 70s GM or an aftermarket valve. Now it could be a lot of people replacing bad ones out, or thinking they are bad so they replace them, but i doubt it. My brother did a mild refresh on a low mile 1987 monte SS. I need to ask him what was in his car. He bought it in N.J. so it may been built in your neck of the woods so to speak.