I thought it might be a good post to talk a bit about brake fluid and show the process of flushing your brake lines. Obviously brakes are a critical system to your bike and some care needs to be taken on them at regular intervals.Something I have discovered in the Official Harley Davidson Service Manual for my 2012 Street Glide is that every maintenance point is not listed on the 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.
The average rider from what I have seen might put 8-12 K miles on their bike yearly. This number might be disputed by some readers and that’s fine, but this is the average that is clocked on the odometers from the 900 or so members of our local HOG Chapter per year. This means their not riding enough to wear their brake pads down enough to need them replaced every year or even every 2 years. If the brakes aren’t being replaced, you can almost be sure the fluid isn’t being flushed either. Now put 2 years, around 20 K miles on the same bike, it still doesn’t need brake pads but now the fluid needs replaced. If the owner doesn’t do it themselves or take it in and specifically pay to have the lines flushed and refilled with new fluid they may begin to experience a difference in the braking of the bike. So even if your not changing out the brake pads, you still need to keep up with flushing the lines and refilling them with new brake fluid. This maintenance point should become a regularity at the minimum of every 2 years and be taken as seriously as changing your oil.
Why brake fluid needs to 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, the brake fluid will begin to absorb moisture from the air. Even once it’s sealed in your system (though it’s never truly sealed) it will continue to absorb moisture from things like rubber seals and hoses. So don’t use a bottle that has had the seal broken because it will have already absorbed enough moisture to affect the brake fluid.
Over time as more and more moisture is absorbed you will get a build-up of water in the system, and 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. 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, but 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, but 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.
This is why it needs to be changed.