Bundy tube
Bundy tube, sometimes called Bundy pipe, is type of double-walled low-carbon steel tube manufactured by rolling a copper-coated steel strip through 720 degrees and resistance brazing the overlapped seam in a process called Bundywelding. It may be zinc- or terne- coated for corrosion protection. It is used in automotive hydraulic brake lines in cars manufactured in the USA since the 1930s.
A 1969 study by the SAE recommended the replacement of Bundy tube with 90-10 copper-nickel alloy UNS C70600 (Kunifer pipe) because of corrosion concerns. Kunifer pipe has since been adopted by European automakers Volvo, Rolls-Royce, Lotus Cars, Aston-Martin, Porsche, and Audi. Bundy pipe retains the advantage higher rigidity, which means less volume expansion under pressure.
The Bundy Tubing Company, started in the USA, was bought in the 1950s by what is now the British company TI Automotive.