Paying Attention To Brake Fluid

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Paying Attention To Brake Fluid

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Harley Davidson Forums >> Do It Yourself Help Forum >> Chassis, Suspension, Brakes, and Front End/Controls >> Paying Attention To Brake Fluid

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Paying Attention To Brake Fluid - 2/26/2006 1:18:20 PM

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Brakes
Fluid Dynamics
Paying Attention to Brake Fluid
By Jim Rizzo

Automotive braking is, as we all know, a really important factor in performance car building. The forward thinkers pay an equal amount of attention to stopping as they do to making their vehicles go fast. But, here's a fact that not too many realize: when you step on the brakes, you command a stopping force that's 10 times as powerful as the force that puts the car in motion. Hard to believe isn't it? No wonder the performance brake market has been vying for and finally receiving our attention. But now that they've educated us in the importance of quality components, it's now time to pay attention to the fluid that activates and allows these awesome components to work.

It only makes sense that after you've spent big bucks and a lot of time upgrading your vehicle, the completion of your job must go beyond adding just any old fluid into the system. It's extremely important to always use a high quality fluid. Let's face it, that quart of Elmo's Wonder Brake Fluid and Windshield Washer Solvent combo you picked up at the swap meet may end up as the only thing between the ball of your foot and a brick wall. So, let's at least pay a fraction of the attention spent on choosing components on choosing brake fluid. The following information should help you better understand the differences between various types of fluids.

FLUID REQUIREMENTS
Brake fluid must function through heat, cold, and all the variations of temperatures affecting all brake systems. In other words, brake fluid must be and must remain viscous (fluid) and resistant to severe conditions. It must also absorb water without having an affinity for doing so. Brake fluid must be pure and contain no contaminants, which will have a harmful effect on rubber components. It also should serve as a lubricant for the moving parts in a brake system and be able to withstand time and service--in other words, its boiling point and chemical properties must remain stable. One more important factor is that brake fluid should cause an electrolytic action that will decompose or degrade metal brake parts.

FLUID CLASSIFICATIONS
These factors are taken into consideration by the Department of Transportation, National Highway Safety Administration in Federal Motor Vehicle Safety Standard 116. FMVSS116-classified fluids fall into three categories: DOT 3, DOT 4, and DOT 5. DOT 3 meets the minimum specifications, while DOT 4 and 5 meet more stringent requirements. The purpose of these requirements is "to reduce failures in the hydraulic brake systems of motor vehicles which may occur because of the manufacture or use of improper or contaminated brake fluid." In addition to setting the minimum wet and dry boiling points for all brake fluids, Federal Standard 116 deals with fluid viscosity, temperature and chemical stability, corrosiveness, water tolerance, compatibility (between formulations), and effects on brake system components.

OK, WHAT'S THE DIFFERENCE?
It seems logical that if DOT 3 is good enough then DOT 4 must be better. And, if DOT 4 is better, then DOT 5 must be better still. OK then, why? It all boils down to boiling point. When brake fluid begins to boil (from the heat that's generated in the system during braking) it forms gas bubbles. As these bubbles collect and grow, they form pockets of gas in the system. And since gas is much easier to compress than fluid, these pockets cause the pedal to easily compress or go soft. This is what as known as a spongy pedal--a condition that causes the heart to race.

BOILING POINTS
The following chart shows the aforementioned FMVSS116 minimum boiling points. These boiling points are the biggest difference in the fluid classifications. You'll notice there are two points for each classification of fluid. The dry boiling point is for fresh fluid that has not yet absorbed moisture. The wet boiling point is for fluid that has been