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I think you are correct that copper brake lines are not popular in America. I don't know how valid the objections are. Perhaps another reader knows.
Bill C.
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Several materials have been / are being used for brake piping.
First & worst is regular copper. Copper work hardens with vibration & flexing & fatigue cracks. Not good for brake or fuel lines.
Low carbon annealed steel has been used for many years. It is not subject to fatigue failure but has rust & corrosion problems.
Various coatings have been used to protect the steel, such as, copper & tin plating. A typical brand is "Bundy"
A 90% copper - 10% nickel alloy (UNS C70600) has been used in Europe for many years. It has the fatigue resistant qualities of steel & very high corrosion resistance. I don't know whether present US car manufacturers & racing organizations are It's use or not.
The confusion may be between copper, copper plated steel, & 90/10 copper - nickel.
D