Brake fluid absorbs water which decreases its boiling temperature significantly. Even 5% water content can cut the boiling temperature in half and causing extreme loss of braking power during high performance/high heat events.
If the brake fluid gets low enough, air gets into the master cylinder. Braking performance is affected if air gets in the brake's hydraulic system decreasing stopping power and can risk your safety behind the wheel. If this happens the brake pedal will get soft or spongy when compressed.
Check for leaks at the bleeder screws, and brake lines at the calipers, and ensure that all connections and bleeders are tight. Top off fluid level and bleed, if necessary.
Street used vehicles and daily drivers should be checked each season and keep an eye out on fluid level every few months. Race cars, and vehicles used at track days and performance events should be checked before every track event.
Brake fluid that is clear or translucent is in good condition. This is best for maximum performance. If brake fluid is cloudy, dirty, brown, or rust colored, it should be changed.
For a street car with little performance use every 2 years, or OEM recommendation, should be sufficient. For track day usage, fluid should be replaced every season. Be sure to bleed after every event to get clear fluid at the caliper, and then top off reservoir.
Brake fluid replacement can be done using a power bleeder or manually bleeding the car, filling from the master cylinder reservoir until clear fresh fluid comes out of each bleeder screw on all calipers. This step needs to be done correctly for optimal performance. Take to a certified mechanic, if needed.
Look for the OEM recommendation for street car usage. Dot 3 is typical for commuter cars and has a low dry boiling point. Dot 4 has higher boiling temperatures but needs to be changed more often due to water absorption properties. Always follow factory recommended replacement intervals, at minimum, if DOT 4 is the OEM brake fluid. DOT 5 is a specialty silicon based fluid and not compatible with any other brake fluid.
This is not recommended. However, DOT 3 and 4 can be mixed in an emergency. If mixing has occurred, brake fluid should be replaced as soon as possible.