Interesting facts about brake fluid

When it comes to enjoyment of motorcycle riding, brake fluid is just as important as engine oil and petrol – and every bit as interesting.

Interesting facts about brake fluid
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Brake Fluid Basics

Good brakes are vital for your safety every time you take your motorbike on the road. While many bikers remember to check their brake pads, they often forget that the brake fluid in a hydraulic braking system needs to be changed regularly. Why?

Most brake fluids contain glycol, which is hygroscopic. This means that over time it will absorb moisture from the air – even in a closed system. This has the effect of lowering the boiling point (as you know, water turns to steam at temperatures above 100°C). When you brake, the friction of the brake pad rubbing against the disc produces considerable heat – just think of how much you use the brakes on a mountain descent. If there is too much moisture in the brake fluid, the intense heat can cause vapour bubbles to form. These bubbles are not able to effectively transport the pressure in the hydraulic system, so the brake pressure point is lost. In other words, the brake pedal or hand lever goes soft and the brakes fail. You then have to pump the lever or pedal until you feel a pressure point again.

Important: Brake fluids are classified as special waste and must not be disposed of with old oil, in the ground or with wastewater. You should therefore dispose of your brake fluid via your supplier, a Louis retailer or a special waste depot! As brake fluid also corrodes a number of plastics, it's best to store old brake fluid in the original container. Once opened, always store your brake fluid out of the reach of children with the lid firmly closed in a dry place which is not subject to temperature fluctuations.

Please note the exception: hydraulic oil must be used in this case.

Please note the exception: hydraulic oil must be used in this case.

For this reason, in the interest of safe biking, it is essential to change glycol-based brake fluid at regular intervals, as specified by the manufacturer (every 1-2 years). If you're not sure how long the brake fluid has been in the system, there are testers you can use that will tell you. 

If you do not have a tester, the colour of the fluid also gives you an idea of how old it is: new glycol-based brake fluid is a transparent yellowy colour, and the older it is, the darker it looks. Opaque, dirty brown fluid should always be changed. This rule also applies to the hydraulic fluid in a hydraulic clutch, which is generally also filled with DOT 4 brake fluid, but this can also be special hydraulic oil. In this case, you should check the cap of the fluid reservoir.

It all depends on the DOT class, and there are many different types.

It all depends on the DOT class, and there are many different types.

DOT classes

The American Department of Transportation (DOT) has specified the following DOT classifications for glycol-based brake fluids for vehicles:

  • DOT 3: Wet boiling point approx. 140°C 
  • DOT 4: Wet boiling point approx. 160°C 
  • DOT 5.1: Wet boiling point approx. 180°C 
  • Racing formula: even higher wet boiling point (values provided by manufacturers tend to vary slightly). 

These days, DOT 4 is by far the most widely used glycol-based hydraulic fluid in the automotive sector, and DOT 3 fluid is now rarely used due to its inferior quality. The brake fluids previously mentioned can be mixed, which means that you can, if you wish, to change from DOT 4 to the higher quality DOT 5.1 for example (not DOT 5!) fluid, and the system would not necessarily have to be completely flushed to do this, although you would obviously achieve the best effect by doing so. Generally speaking, you can replace DOT 3 with DOT 4, although caution is advised if you have a classic motorbike, as DOT 4 has proven somewhat more aggressive on rubber parts than DOT 3 (if in doubt, consult a classic motorbike specialist).

All glycol-based brake fluids are poisonous and corrode materials such as paint, plastic and several types of rubber. Metal surfaces can also be affected. You should therefore proceed carefully when working with brake fluid and immediately wipe away drips or spilt fluid and follow this up by rinsing with water.

In addition to the glycol-based hydraulic fluids previously mentioned, DOT 5 brake fluid (not to be confused with 5.1!) is also commercially available. This fluid is silicone-based and is used in many Harley-Davidson and Buell motorcycles, in particular. Never mix this fluid with any of the fluids previously mentioned, as this would cause flocculation (clumping) and clogging of the system.

Silicone-based brake fluid is not hygroscopic and can therefore be left in the system for longer (depending on the vehicle manufacturer's specifications). To switch a DOT 3 or DOT 4 braking system to DOT 5 you would first have to thoroughly flush the entire hydraulic system, and also obtain approval from the vehicle and/or brake manufacturer.

Please note: Silicone-based brake fluid does not corrode materials, but is extremely poisonous. Always wash your hands after working with brake fluid and make sure it does not fall into the wrong hands.

Check cap for correct brake fluid

Check cap for correct brake fluid

As a rule, the correct brake fluid is indicated on the original cap of the brake fluid reservoir. If in doubt, check your maintenance manual or ask your local motorcycle dealer.

Our recommendation

The Louis Technical Centre

If you have a technical question about your motorbike, please contact our Technical Centre, where they have endless experience, reference books and contacts.

Please note!

These tips for DIY mechanics contain general recommendations that may not apply to all vehicles or all individual components. As local conditions may vary considerably, we are unable to guarantee the correctness of information in these tips for DIY mechanics.

Thank you for your understanding.

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