Why do a brake flush?
It is required to do a brake flush every two years. The reason being, that not only does brake fluid get dirty over time, but it is also hygroscopic. This means the fluid draws moisture (water) into the brake system. Over time, this compromises the braking efficiency. The images show damage to the brake booster and brake master cylinder of a vehicle which stood outside, without the components being closed or covered. It allowed the brake fluid to attract moisture without delay, and eventually the water content was so high, it caused the components to rust. What is usually a clear liquid (both brake fluid and water), turned into a texture similar to a muddy goo.
On older BMW’s and MINI’s, you will notice a little clock on the instrument cluster, reminding you every second year to have a brake flush done. On the newer models with condition-based servicing, the brake flush is grouped together with the service items. This can usually be seen listed with other service items in the service info menu on the cluster.