They say a picture is worth a thousand words…
Brake fluid change is a topic that has been generating discussion lately when previously, most riders ignored the service interval in the owner's manual. The debate really took off when Harley was forced to recall motorcycles due to water getting into the ABS unit freezing the pistons, and now everyone recommends the two-year/24k miles replacement of fluid regardless of miles or condition of the existing fluid.
Let me start with some simple facts. Dot 4 Brake Fluid has a “dry” boiling point of 446 °F. Dry refers to fresh, brand new brake fluid. It has a “wet” boiling point of 311 °F. Wet is when it has absorbed 3.7% water by volume. Basically a few drops of water/moisture can drop the boiling point drastically.
Now let's talk Harley-Davidson motorcycles. Many of the rear brake master cylinders are located directly next to the exhaust, which we know is hot, then add that brakes can develop a substantial amount of heat, particularly when used repeatedly, and you get to at or above the boiling point very quickly.
So now that we have a little understanding, add in that if you remove the cap covering the master cylinder reservoir you introduce air which contains moisture. Plus, water can get in through the microscopic pores in your hydraulic lines. Once water gets into the system it can theoretically float to the top and to the highest point in the system, which many times is the ABS unit. Once there, it leaves deposits that can stick to the valves that actuate the ABS system defeating its purpose. On older non-ABS systems this water typically traveled to the master cylinder which is the highest point in the system which is why this really wasn’t such an issue in pre-ABS motorcycles.
Much more can be written about this topic but the safe choice is to change it every two years/24,000 miles. We also recommend it get changed any other time work is performed in the brake system.