THE WILD AND WEIRD WORLD OF BRAKE FLUID
It was no easy chore for the braking industry to find a fluid that had the appropriate properties to handle the stressed conditions of a hydraulic braking system. The fluid needed to be light enough that the inner workings of the brake hydraulics would respond instantly to modulation, but compatible with plastic, metal and rubber components. Most important, the fluid had to remain consistent in temperatures that vary from 500 degrees (much hotter than the transmission) all the way down to freezing...and still be able to transfer through the brake line into the caliper pistons.
It's due to high heat that one cannot simply fill the master cylinder with 30 weight motor oil. Regular oil would expand to the point of exceeding the relief space inside the master cylinder reservoir until the brakes locked up. In cold climates, standard oils will also thicken and cut the fluidity of the movement.
The fluid answer came in the form of a Polyglycol, a fluid that stayed consistent from hot to cold and had the ability to lubricated the moving pistons and seals.
But brake fluid is much more complicated that all of that.
WHAT IS A HUMECTANT?
Try this experiment at home. Fill a cup to the brim with DOT 3 brake fluid and sit it outside on an unpainted surface. Within a short time, the brake fluid will overflowed.
So what? you ask.Not only have you waisted a whole lot of brake fluid ,but also what you have just witnessed is a brake failure in the making. Why? Read on.
Brake fluid is not the same as other fluids (like motor oil). Hydraulic brake fluid must remain stable under high temperatures, provide lubrication for moving parts, seal the system and have a light enough viscosity to react quickly. Polyglycol fits the brake fluid profile to a T.
Unfortunately, glycol, an alcohol derivative, has one nasty habit. It absorbs water. In scientific terms it is a humectant; a substance that promotes the retention of moisture.
Which brings us back to our experiment. Why did our cup of brake fluid overflow while just sitting there? The brake fluid absorbed moisture out of the atmosphere, dispersed it throughout the brake fluid and increased its volume. Brake fluids ability to absorb moisture from the atmosphere is the cause of 95% of hydraulic brake failures.
WHAT IS BRAKE FADE?
Using two small brake pucks to stop a 400 rpm, ten-inch disc that is powered by a 50 horsepower motor generates heat. How much heat? Up to 500 degrees. This heat is transferred through the brake pads directly to the brake fluid. Even in a perfect state (with no water contamination) brake fluid can become so hot that it will boil. When it boils, it vaporizes into a compressible gas. Under braking, the vaporized gas will compress before the hydraulic fluid, and if there is any lever play left the pucks will compress later. Maybe too late! Your brakes get mushy, the lever goes slack and corners start coming at you fast. Bad news!
Keep in mind that we have yet to consider that the fluid has been contaminated with water. If you kept notes in high-school physics you will remember that water boils at 212 degrees, much lower than brake fluid. When water gets in your brake fluid it lowers the boiling point and causes the brakes to fade even earlier. This dilemma is known as vapor lock. A more proper term would be vapor fade since the gas produced by the boiling brake fluid rarely expands to the point of locking motorcycle brakes on.
HOW DOES WATER GET IN?
Hydraulic brake systems are sealed, so how does water get in? There is a good chance that the brake fluid was contaminated with H2O before you put it in your brakes. Or the moisture entered when the reservoir cover was off. It's even possible for hydroscopic fluids to pull moisture past the master cylinder diaphragm. As a matter of fact, cheap brake fluids are so hydroscopic (water attractive) that manufacturers have started to use Teflon brake hoses to keep moisture from being drawn through the rubber.
WHAT IS DOT?
DOT is the acronym for the Department of Transportation. The DOT administers fluid regulations and monitors the quality of the brake fluids being sold. The DOT criteria is so high that there are only three companies in the United States that produce brake fluid. No matter whose name is on the can (as long as its American-made), you can guess that the brake fluid came from Dupont, Dow or Union Carbide. Europe also has only three fluid manufacturers and companies like Motul use these blends (which do offer a different profile than what is available in America).
WHAT IS THE WET BOILING POINT?
Brake fluid manufacturers, who are proud of their product, will list both the dry and wet boiling points of the fluid. The dry boiling point (which is the higher number) is taken when the fluid is in perfect condition. The wet boiling point is taken with the fluid fully hydrated and at equilibrium. The higher the wet boiling point the less susceptible the fluid is to absorbing water.
WHAT ARE CINITASTOKES?
The viscous performance of the fluid is also tested at hot and cold temperatures, and is measured in cinitastokes (cst). A lower cinitastoke count indicates a thinner viscosity. The thinner the brake fluid the more responsive it is to the hydraulic pressure emanating from the master cylinder. This is good because thicker fluids are slower to transfer energy from the lever to the pucks.
WHAT IS DOT 3?
In almost all cases DOT 3 is the least expensive and lowest performing brake fluid produced. The exception to this rule is Maxima DOT 3 550 with a 568 dry boiling point (the highest of any stateside-produced motorcycle brake fluid).
Base: Synthetic Polyglycol base.
Dry boiling point: Minimum required dry boiling point-401 degrees.
Wet boiling point: Minimum required wet boiling point-284 degrees.
Viscosity: Viscosity at minus 40 degrees-1500 cst.
Compatibility: DOT 4 & 5.1.
Color: Amber to clear.
WHAT IS DOT 4?
Most DOT 3 fluids meet DOT 4 regulations and vice versa (save Motul's Racing Brake Fluid 600, which has incredibly high dry and wet boiling points of 585 and 421 degrees). Motul 600 exceeds DOT 3's 1500 cst viscosity rating by two percent (at minus 40 degrees) to be labeled as such.
Base: Synthetic Polyglycol base.
Dry boiling point: Minimum required dry boiling point-446 degrees. Wet boiling point: Minimum required wet boiling point-311 degrees. Viscosity: Viscosity at minus forty degrees-1800 cst.
Compatibility: DOT 3 & 5.1.
Color: Amber to clear.
WHAT IS DOT 5?
The high boiling points of DOT 5 made DOT 5 the hottest fluid going in the late 80s (now only Harley-Davidson specs it). But the fluids syrupy nature wreaked havoc on the system if water or air entered. Since thick brake fluid won't allow microbubbles to float to the top of the reservoir, unwanted air is dispersed throughout the system (making lever action mushy). Unlike a Polyglycol fluid, silicone-based brake fluids do not absorb water. Instead, the heavier water settles toward the bottom of the hydraulics, where the caliper resides. That means a liquid with a low boiling point is residing in the area that generates the most heat. If not bled, the caliper can also corrode.
Base: Silicone base.
Dry boiling point: Minimum required dry boiling point-500 degrees. Wet boiling point: Minimum required wet boiling point-500 degrees. Viscosity: Viscosity at minus forty degrees-900 cst.
Compatibility: DOT 5 silicone base.
Color: Purple. The purple color is a DOT mandate indicating a silicone base that cannot be mixed with the yellowish DOT 3, DOT 4 or DOT 5.1 fluids. When mixed, the fluids coagulate and turn to Jell-O. Unlike DOT 3, DOT 4 and DOT 5.1, DOT 5 will not damage painted surfaces.
WHAT IS DOT 5.1?
This European-produced fluid came into existence due to the fast moving, anti-lock braking systems (ABS) used on todays automobiles. DOT 5.1 is the thinnest fluid and offers the least hot-to-cold change in viscosity. This fluid has the highest wet boiling point of any of the synthetic Polyglycol.
Base: Synthetic Polyglycol base.
Dry boiling point: Minimum required dry boiling point-500 degrees. Wet boiling point: Minimum required wet boiling point-356 degrees. Viscosity: Viscosity at minus forty degrees-900 cst.
Compatibility: DOT 3 & DOT 4.
WHAT THE FACTORIES USE?
Although Team Honda, Kawasaki, Suzuki, Yamaha and KTM will use the best performing fluids produced, you can expect your motorcycle to be floored with low cost DOT 3 fluid. It is best to replace it after break-in with a racing DOT 3, 4 or 5.1 fluid (preferably with one that carries the highest dry boiling point you can find).
There are several important tips to follow when dealing with brake fluids:
(1) Fluids with high dry boiling points but low wet boiling points need to be changed often.
(2) Correctly changing brake fluid means completely flushing the system (gravity feed).
(3) Mixing new fluid with the old immediately lowers the boiling point of the new fluid.
(4) Although fluids with high wet ratings can run longer before water contamination becomes too great, it's never smart to run fluid longer than six months.
(5) Remove fluid spilled on disc rotor and pads with contact cleaner immediately.
6) Although brake fluid is packaged in a hermetically-sealed environment, it's still not a good idea to purchase a dusty container of fluid that looks like it's been sitting on the shelf for years.
(7) Seal the container immediately after use to keep moisture retention at bay.