Wikipedia to the rescue.
"Most brake fluids used today are glycol-ether based. Brake fluids must not corrode the metals used inside components such as calipers, wheel cylinders, master cylinders and ABS control valves. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this." I'll interpret this statement - the brake fluid does not corrode the metal - the entrained water does. I couldn't find an explanation why glycol-ether softens paint, but I've observed that it's a fairly slow reaction. I had to clean and repaint a control arm and coil spring because I had a leaky copper washer on one of my new calipers. Wash with water, then scuff beyond the damage with 400 grit, then clean with brake cleaner and repaint.
"Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years under non-racing conditions. Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Electronic testers and test strips are commercially available to measure moisture content, however moisture test strips were taken off the market because they absorb moisture in the air before they can be used. The corrosion inhibitors also degrade over time. Degraded inhibitors cause corrosion in the braking system. The first metal to corrode is copper. You can determine when it is time to replace brake fluid when copper ions hit 200ppm. New fluid should always be stored in a sealed container to avoid moisture intrusion."
"DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climates, particularly in Russia and Finland."
Silicone can cause OEM brake rubber materials to swell. Ever get brake cleaner on nitrile gloves? They soften then fall apart. This is what can happen if you use DOT5 with old school brake components.
The best bet is to stick with good DOT3 from a sealed container, and bleed and flush every 2-5 years if you want to keep the car. You can crack open each bleed screw one at a time, attach a short hose to a collection cup, and keep topping off with the fluid level in the master cylinder. This way you have fresh fluid without introducing air into the system.
