Periodic Replacement is Required | How to Replace Brake Fluid that Has Degraded Over Time?

01-3.jpg How to

The components of brake fluid used as hydraulic fluid for disc brakes are glycol- or silicone-based liquids with a high boiling point, low viscosity, and excellent long-term stability. It is transparent when new, but turns brown as it deteriorates. Not only does the color change, but performance also deteriorates, so regular replacement is required every two years regardless of mileage.

The old Harleys used to be silicone-based DOT5, but these days they use DOT4

02-4.jpg The type of brake fluid to be used is specified in the service manual and often on the caution seal in the reserve tank. 2008 Sportsters are already DOT4 rated. The 2008 Sportster is already DOT4, so silicone DOT5 will not work on this model year.

Until the early 1980's, the main standard for brake fluid in disc brake vehicles was DOT3, but since then, DOT4 has become the mainstream standard. DOT stands for Department of Transportation, which is the United States Department of Transportation. DOT stands for Department of Transportation, which refers to the U.S. Department of Transportation, and one of its standards is for brake fluid, which is used as a yardstick for brake fluid development worldwide. Commonly available brake fluids include DOT3, DOT4, DOT5.1, DOT5 standards are available, and are distinguished by their main ingredients, boiling points and viscosities.

The majority in motorcycles are DOT3 and DOT4, which have glycol as their main component. In contrast, the main component of DOT5 is silicone! The key performance requirement for brake fluid is a high boiling point. An important performance requirement for brake fluid is a high boiling point. The temperature of the brake pads can reach over 300°C during sports driving on the circuit, and even higher when braking so hard that the rotors turn red during automobile racing.

When the heat heats the brake fluid in the caliper from the pads through the caliper piston, if the brake fluid is tap water, it will boil at 100℃. If the brake fluid is tap water, it will boil at 100 degrees Celsius. This will cause the gases that are dissolved in the water at normal temperature to expand and form air bubbles, which will cause a vapor lock condition as if air is trapped in the hose when the brake lever is squeezed.

DOT3, which uses glycol, has a dry boiling point of 205℃ and a wet boiling point of 140℃, while DOT4 has a dry boiling point of 230℃ and a wet boiling point of 155℃. In contrast to DOT3 and DOT4, which are made of glycol, DOT5, which is made of silicon, has a dry boiling point of over 260℃ and a wet boiling point of over 180℃. However, glycol and silicone are completely different in composition and characteristics, so they cannot be mixed together. Also, the rubber seals inside the master cylinder and calipers are special products, so you cannot use them by draining the fluid completely and replacing it.

In the old days, Harley-Davidson used DOT5 brake fluid, which is silicone. However, since the mid-2000's, the fluid has shifted to DOT4 and DOT5 cannot be used on DOT4 specified models. Of course, using DOT4 fluid on a DOT5 era Harley is also strictly prohibited.

POINT
  • Point 1: Brake fluid specifications are defined by dry and wet boiling points.
  • Point 2: DOT3 and DOT4, whose main component is glycol, and DOT5, whose main component is silicon, have different compositions and characteristics, so mixing them is strictly prohibited.

The boiling point of brake fluid is lowered when moisture is mixed in.

03-4b.jpg Suck up the brownish fluid in the reserve tank with a dropper. If you pour new fluid into the dirty fluid, it will take a long time for the fluid to become clear and the new fluid will be wasted. Therefore, it is more efficient to drain the old fluid as much as possible. However, when the tank is empty (especially at the front), the piston of the master cylinder pushes air and it becomes air bite, so do not drain it completely.

04-4b.jpg Fill the reserve tank with new DOT4 fluid. DOT4 has a higher boiling point than DOT3, but external products use DOT5 which has an even higher boiling point than DOT4 even with glycol..1 standard fluid exists. This fluid has a dry boiling point of over 260℃ and a wet boiling point of over 180℃, which is equivalent to silicone DOT5. You can change it to 1, but you can't change DOT4 to DOT5. It's complicated, but I'll keep it in mind.

The main causes of brake fluid deterioration are heat and moisture, and glycol, the main component of DOT3 and DOT4 fluids, absorbs moisture from the air. The main ingredient of DOT3 and DOT4 fluid, glycol, absorbs moisture from the air, not only from rainwater entering the reserve tank, but also from moisture coming in contact with the master cylinder cap through the vents. The wet boiling point refers to the condition where 3.7% of water by volume is mixed into the new fluid.

Whether it's only 3.7% or as much as 3.7% depends on your personal feeling, but the website of a brake parts manufacturer explains that fluid after 1-2 years from new contains this level of moisture, so 3.7% is not an extreme number. What is surprising is the difference between dry and wet boiling point, after just 1-2 years, the boiling point drops almost 60℃. Still, it's over 100℃, so it's better than water, but I shudder to think how much the boiling point will drop if it absorbs more moisture.

Some riders may think that because they are not going as fast as on a circuit, they are safe. However, in situations where the engine brake and front/rear brakes are used together, such as long descents on winding roads, the fluid temperature may rise due to the heat generated in the brakes even if the speed is not high. Also, the idea that it is safe to use glycol because it has been used for a long period of time but has only been used for a small number of miles will not work due to the hygroscopic nature of glycol.

POINT
  • Point 1: Glycol has hygroscopic properties and absorbs moisture in the air even when not riding in the rain, causing it to change.
  • Point 2: The boiling point of brake fluid drops by nearly 60℃ after just 1 to 2 years of use.

Replace the caliper pistons and master cylinder every two years to check them as well.

05-4b.jpg Loosen the bleeder plug of the caliper, and suck out the old fluid in the caliper and hose with a syringe (syringe). At this time, the amount of fluid in the reserve tank will drop, so work while observing the tank to avoid emptying it. If the fluid has been emptied after replacing the brake hose, it is effective to fill the caliper and master cylinder with new fluid by pumping it backwards. It is necessary to work carefully.

06-5b.jpg After draining the fluid from the bleeder plug, make a coyote out of tissue paper and insert it into the plug hole to suck up the remaining fluid inside. You can also use a parts cleaner nozzle to spray the fluid, but in any case, do not leave any fluid inside. Brake fluid is hygroscopic, so if it remains in the plug, it will cause the plug itself to rust. It is also important to cover the plug with a rubber cap to prevent water from entering the plug. If you find one that is cracked or missing due to age-related deterioration, you should purchase and install one.

Riders who regularly have their motorcycles serviced or repaired by a motorcycle shop, or DIYers who change their own fluid, should be fine, but riders who pay attention to the level of brake pads but not the brake fluid should be careful. Brake fluid deteriorates just like engine oil and needs to be changed regularly.

If you look at any manufacturer's service manual, the replacement schedule is every two years, so for 250cc and larger motorcycles with an inspection, it is easy to remember to replace at the time of vehicle maintenance. For mopeds and models below 250cc, there is no obvious trigger, so it is best to use the calendar or reminder function on your smartphone to reserve the maintenance timing. There is a possibility that you will change your model in the meantime, though.

When changing the brake fluid, we not only change the fluid but also check the overall health of the brakes by cleaning the remaining pad level and the area around the caliper piston and checking the condition of the rubber seals. At this time, if you find the caliper dust seal protruding from the caliper groove or the caliper itself is wet with brake fluid, consider a caliper overhaul necessary. When fluid seeps from a damaged caliper seal, it is often the area around the caliper piston and pads that gets wet first, not the outside of the caliper, so check carefully before changing the fluid.

If there is no problem with the caliper and you only need to change the fluid, you can reduce the amount of time it takes to bleed the fluid by replacing the brake hose without introducing air into it. To do this, suck up as much of the old fluid as possible with a dropper, taking care not to expose the port at the bottom of the reservoir tank (the hole that feeds the fluid to the master cylinder), and then pour the new fluid into the tank. This way, the new fluid can be pumped in immediately when the brake lever is held, and the old fluid in the hose and caliper can be expelled efficiently.

If the fluid you see through the window of the reserve tank is brown like tea or whiskey, replace it even if it has not been two years since the last change. Remember that brake fluid should always be kept clear to maintain its performance.

POINT
  • Point 1: It is hard to forget if you decide to change the fluid of the motorcycle with the vehicle inspection at the timing of the vehicle inspection maintenance.
  • Point 2: By injecting the fluid so that the reserve tank of the master cylinder is not empty, the fluid can be replaced without air bites.
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