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Originally Posted by Kestas
Brake lines are not formed from traditional seamless tubing but are formed in a unique way. A thin strip of steel is spiral wound in a tight diameter (hence, the earlier reference to paper roll tube) and the thin gaps are filled with an copper-rich alloy identical in composition to a brazing alloy.
The outside of the tube is then coated with terne plate, which is a lead-rich coating used for corrosion resistance. The inside is not coated. The braze material is naturally open to the brake fluid inside the brake line.
I imagine that, over time, water gets absorbed into the brake fluid and an electrolytic cell develops, moving the copper ions around from the brake lines to other parts of the system that may be highly electrolytic, such as the corrosion pits in steel, where the copper drops out of solution.
Given all the other good stuff on the internet, it's interesting that there's nothing written on bundy tubing, nor can I find any reference in my steel or automotive handbooks.
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Well, this is new information to me. The only example I have around is the original front to rear brake line from my 63 Corvette, which appears to be seemless steel tubing with a zinc or cad plate. I recall fabricating a new line from what appeared to be essentially the same material that was sold as "brake tubing".
I'm not familiar with this tubing construction technique. Is it unique to some manufacturers? Why is this type of construction used? I take it the name of this type of tubing is "Bundy tubing"?
The brake lines on my 190 have a dark green finish on them. I assume it's some kind of paint and that the tubing itself is plated.
If copper is a primary source of corrosion in brake systems then maybe this spiral wound copper brazed material should be replaced with something that doesn't have any copper content.
I'm mystified.
Duke
P.S. Did a little googling on "bundy tubing" and "brake tubing". The following pages are interesting.
http://www.copper.org/applications/automotive/brake.html
http://www.cda.org.uk/Megab2/corr_rs/is49/default.htm
It appears that a new brake line material is now being used by some OEMs that is about 90 percent copper and 10 percent nickel, so I suppose that it must not have the work hardening problems of common copper tubing. It's primary advantage is greater external corrosion resisitance than plated steel tubing, but maybe it increases the propensity for internal corrosion because of the copper content. "Bundy tubing" appears to be a rather old construction method, but may be used for some brake tubing applications.
I would not expect the new copper/nickel tubing to be magnetic due to the very low nickel content, however I checked, both the old line from my Corvette and the lines on my 190 and they are both strongly magnetic indicating to me that they are steel.
Duke