Brake Fluid Drain: why, when, how
Changing brake fluid can be a slippery subject. Some manufacturers include it in their maintenance schedules and others don't.
Mercedes-Benz, for example, says brake fluid should be replaced every two years or 20,000 miles, and Volkswagen says that should be done on most of its models every two years regardless of mileage. Subaru recommends fresh brake fluid every 30,000 miles. On the other hand, most Chevrolets can go 150,000 miles or 10 years, according to Chevy's maintenance schedule, and many Ford, Chrysler and Toyota vehicles don't list brake fluid as a regular maintenance item.
Check your car's owner's manual to see what the manufacturer recommends. You might also want to discuss the slippery subject of brake fluid with a trusted mechanic if the manufacturer doesn't give any guidance. Don't be surprised if a mechanic suggests replacing the brake fluid periodically, because mechanics probably have seen what can happen if you don't.
What can happen? Even though brake fluid dwells in a sealed system it still can absorb moisture over time, and that can lead to corrosion in the brake system. Moisture also lowers the boiling temperature of brake fluid, and that can reduce braking effectiveness in repeated hard stops. Eventually it will lead to a "dead pedal" where the pedal falls to the floor and you cant stop. Good luck with that! Or on the opposite side of things, moisture in the line will cause steam to build up, which can cause the pressure to push up against the brake pad, wearing it & the rotor out prematurly. that too can cause an overheated braking system, and a fire hazard
Put it this way, brake fluid is as important as seatbelts & airbags. Its the 'blood' of the system. I would have it drains & replaced every year or two. Its litteraly two, $5 bottles of fluid, and about 30 mins of your time. A shop can do it, probably for $50. Tell me $50 isnt worth your family's safety?