Brakes are a critical system for your bike, and some care needs to be taken on them at regular intervals. I have discovered that every maintenance point is not listed on the service maintenance interval chart. What? You would think it would be, but it's not. Brake fluid is one of those maintenance points they do not list. I can only speculate as to why they would leave this critical point off.
Frequency Of Brake Fluid Changes
Some people don't ride enough to wear their brake pads down enough to need them replaced every year. If the brakes aren't being replaced, you can be sure the fluid isn't being flushed either. This maintenance point should become a regularity every two years and be taken as seriously as changing your oil.
Why Brake Fluid Should Be Changed
Glycol-based brake fluid is hygroscopic. This means that the fluid will aggressively absorb moisture. As soon as the seal is broken on the bottle, brake fluid will begin to absorb moisture from the air. Even once it’s sealed in your system, it will continue to absorb moisture from things like rubber seals and hoses. Don't use a bottle that has had the seal broken. It will have already absorbed enough moisture to affect the brake fluid. Over time as more moisture is absorbed, you will get a build-up of water in the system. It is the water that changes the performance characteristics of the brake fluid.
Fluid in its natural state is not appreciably compressible, but when that fluid is vaporized by high temperatures, the resulting vapor can be compressed instead. This means the fluid is then less able to convert force into pressure. It is for this reason that brake fluids have very high boiling points to combat this. When moisture is absorbed into the fluid, the boiling point drops dramatically. This can cause your brakes to have a squishy feel. You may even have to push harder on the brake pedal or pull the brake handle in further to get a brake response.
DOT 4 brake fluid has a boiling point of around 445 degrees Fahrenheit. Once the fluid absorbs enough moisture to the point where it is then made up of just a few percent water, the boiling point can drop as much as 175 degrees F.