Rust. I have an '03 that I absolutely love driving, no exterior rust and the bottom of the truck looks really clean, yet I still had a brake line blow out a couple weeks ago. The good news is that Summit sells a complete prebent stainless kit for them cheap, mine was $75. Between Summit and Rock Auto, I replaced all lines, calipers, pads and rotors for like $350 in parts. I noticed the fuel lines near the drivers door looked a little crusty where the plastic clips are, looked fine elsewhere. I'm gonna hit them with some POR15 and hope they're good to go.