For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity. Since your vehicle is exposed to a variety of temperatures, over time, brake fluids can absorb water. 
 

    This water can boil from heat build up caused by excessive braking. The presence of water in the brake fluid can cause a loss of braking and the corrosion of brake components. The components most affected by this include the master cylinder, wheel cylinder, callipers, brake lines and ABS components. Most manufacturers recommend service every 50,000km or every 2 years, which ever comes first.