You didn't mention how old your vehicle is or when the brakes were last serviced. Larry is correct -- you should bleed periodically. Brake fluid (unless you run silicone) is "hygroscopic" -- which means it attracts moisture. Over time, brake fluid will pull moisture out of the atmosphere. Obviously, this will be more of a problem in Seattle than Tucson, but that's one way water can get in there. You may never see it, but it can be enough to rust the lines from the inside.