The original brake pipes are steel with some sort of coating. Whatever it is, it corrodes pretty quickly giving a white 'efflorescence'. Many 75/ZTs failed their first MOT on brake pipe corrosion. I wire-woolled all mine after two years and double-coated in Hammerite. After the Hammerite was fully cured, they were then labbered in WaxOyl and I've had no MOT issues thus far.
TC
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