Brake fluid - What is it and what does it do?

By, Dell Gauthreattx

How many times have you decided to go for a ride, and thought that you should change your brake fluid? The answer to this question is going to be never for about 99.9% of motorcycle riders out there. One might give thought to other parts of the motorcycle, or other parts of the braking system, such as the pads or shoes, maybe that broken lever, but brake fluid does not fit into the equation of Rider, motorcycle and road. The truth is that without the fluid, the brake system is a total loss, and bad fluid can in some cases be as bad as none at all.

So lets see if we shed some light on the brake fluid subject, and help you become more aware of just how important this little bit of fluid is to you brake system, and why keeping it fresh paramount to your safety.

What is it?

There are a few different types of brake fluid out there, and all of these fluids are a hydraulic fluid. What does that mean? Hydraulic fluids, are liquids that cannot be compressed, therefore they transmit force from point A to point B. The fluids used for brake systems have been selected due to their characteristics when it comes to heat, and more to the point, the temperature at which it will boil. This is important because while liquids are non-compressible, gasses are, and brake fluid has a very high boiling point. This helps the brake system because as stated before, you cannot transmit force through a gas.

The U-S- Department of Transportation (DOT) has put very strict specifications on brake fluids, to meet federal standards. These fluids must meet or exceed Federal Standard #116. There are three minimal specifications under standard#116, DOT 5, DOT4, and DOT 5.

What are the differences between the fluids?

All of the fluids currently being used are either a Poly Glycol Ether or a Silicone based liquid. DOT 3 and DOT 4 are both a P.G.E. base, while DOT 5 is a silicone-based fluid. Both DOT3 & 4 are extremely Hydroscopic (Means they will mix with water), while DOT 5 is non-hydroscopic. To understand just how hydroscopic DOT 3 & 4 are, tests have shown that over time it will pull water past brake piston seals as well as the master cylinder seal, and over time, through the rubber hoses of the brake system itself. DOT 5 gets most of its water contamination from being heated up than cooling down. All brake systems will eventually become contaminated with water, and therefore the ability to transfer force along the system will be lessened.

Some riders complain of a "Spongy'' feel, when the bike is ridden in a more aggressive manner. This is due to the fluid getting closer to it's boiling point. There are two different boiling points for the fluid, one is a WET boiling point, and one is a DRY boiling point. The Wet boiling point is lower and takes place when the fluid has 3% water by volume. As the fluid breaks down, the braking process will need to start at a further distance from the stopping or turning point. This gradual effect often goes unnoticed due to its slow coming process. This is a problem for riders because it means that you will lose time on the track, and that you might not be able to stop on the street fast enough if a sudden need arises.

What does all of this mean to you as a rider? I like to think that changing your brake fluid every year, or as needed, is a small piece of insurance, which will be there to help you slow down or stop when you need to.

Have a technical question? Ask the professional! Contact our Service Advisor, Dell, today!