I just went through this on my 98 Oldsmobile, brake lines severely coated with rust and one let go, luckily in my driveway instead of the freeway where I drove it the last time.
Age doesn't seem to always be an indicator either, on the Cadillac forums a guy is dealing with GM on a claim for a brake line that let go on his 2008 STS.
After doing a lot of web reading I chose a new brake line that doesn't rust. It goes by the name NiCopp, a nickel copper blend if I remember correctly. It bends very easily without kinking and I was able to use the factory routing over the rear suspension on my Olds. This nickel copper blend is supposedly standard on European cars for over a decade and hasn't had any issues that I could find. The only catch is that it costs about 2.5 times as much as steel line, about $50 for a 25' coil versus $22 for the regular steel lines. I found Advance Auto carries NiCopp.
The only other item to note if you haven't fixed this yet is that you will likely need a bubble (ISO) flare tool to make the correct ends. My 98 uses the bubble flares and I believe the GM trucks do as well. I had to order the tool on-line as nobody stocks anything locally other than the old single/double flare tool.
I plan to get under the Escalade and my other cars to inspect the lines and if I find them rusted I'll be replacing them this spring with the NiCopp stuff.
It's pretty sad that vehicle manufacturers aren't using something more durable on such a huge safety item.
Attached is a pic of the brake line I just replaced, right in the middle is a small dot, that's where she let loose. You can see how badly the line looks though, it's that way all the way from the rear to the front. This spring I'll also be looking into replacing the fuel lines as well as they look just about as bad as the brake lines.
