Removing seized Kelsey Hayes pistons

ppsi said
It looks like I'll be ordering the steel pistons from rock auto. Is there a noticeable advantage to the phenolic?

No such thing as phenolic pistons for the Kelsey Hayes, 4 piston, disc brake calipers, in all the decades that i have been involved in rebuilding this type of disc brake caliper.
Never heard of em, or seen em, in real life.