Today, we'll review how a brake system works by focusing on the various fluids being used. Over the last 30 years, brake systems have grown more complex and sophisticated, evolving from a basic front-wheel disc to a triple disc setup. New braking technologies have also been introduced, such as antilock brakes (ABS) and Honda's DUAL and LBS systems. Composite materials are more and more popular, which explains the radical change in fluid selection. Furthermore, these new technologies have altered maintenance techniques and habits; they now require specialized tools and highly skilled technicians.
The past few years have seen brake fluids turn the technological corner (both in terms of classification and blends). They're more effective than ever at meeting the specific needs (mechanical and chemical) of modern brake systems.
In Canada, the five main types of brake fluid now available are DOT3, DOT4, DOT5, DOT5.1 and DOT5.5. The first two are more widely-used, glycol-based fluids that are compatible with most motorcycles, while DOT5s are silicon-based fluids specifically designed for certain brands of motorcycles. These fluids are categorized based on their boiling point. As a DOT3 or DOT4 brake fluid absorbs water, its boiling point decreases. DOT5 fluids do not absorb water, so their boiling point will remain relatively stable.
DOT5.1 is designed for ABS-equipped motorcycles, while DOT5.5 is exclusively recommended for carbon brake systems. Some sportbikes, mainly used for racing, feature such brakes.
As mentioned earlier, these fluids are developed to meet specific needs based on their reactivity and heat resistance. By the way, the "DOT" letters refer to the U.S. Department of Transportation, which administers the Federal Motor Vehicle Safety Standards (FMVSS).
Compatibility is often the main criteria used by manufacturers to recommend a particular type of fluid. For example, Harley-Davidson uses DOT5, while Italian specialist Brembo relies on DOT4 because the materials that make up its brake components are not compatible with DOT5, so there's only one option. However, if you can't find a DOT4 fluid, you can make do with DOT3 since they're both compatible. Of course, it's always best to use the recommended type of fluid.
Being silicon-based, DOT5 is not compatible with other fluids. Moreover, while you can change from DOT3 or 4 to DOT5, you must bleed and clean every part and hose -- not to mention the master cylinders and callipers -- with ethylic alcohol.