DocStein99:
I was curious about the automotive brake lines too, and can't get a scientific answer. The mechanics just replace brake lines and nobody ever asks about the copper, lead, aluminum content of the pipe. It's soft enough for me to shape using hand tools.
The lines for hydraulic brakes that I have ever worked with are steel. Plated on the outside for corrosion control. They have to withstand tremendous pressure without expanding because your life may depend on them.
Copper tubing WILL expand in use if the fluid has much pressure at all. That is why you use new tubing for any plumbing using copper. Old tubing will be too large for brass fittings.
Payl