All modern motorcycle brake systems use hydraulics. The physics on which they are based are relatively simple: liquids do not compress.

Pressure is generated from the brake levers to the master cylinder. The master cylinder then adds pressure to the brake fluid whick is directly moved through the brake lines into the calipers, pushing the brake pads out against the rotor, creating friction and stopping the motorcycle.

The design of a typical motorcycle front brake master cylinder and lever allows you to generate over two tons of pressure within the brake system. That's a lot of pressure, but that's what is required to stop a spinning brake rotor on a 400 to 800 pound motorcycle at speed.

Since liquids don't compress, the pressure throughout the brake system is essentially equal at any time. The pressure in the master cylinder is the same pressure as in the caliper, and also in your brake lines. That is why you need good brake lines and brake fluid. They have to convey thousands of pounds of pressure from the master cylinder to the calipers.

Great brakes are a BMW hallmark. The current BMW Motorcycle brake systems offer the highest level of stopping power in the industry. So what do you need to do to keep your brakes operating to their maximum potential? Simple, just keep reading;

Brake Fluid: In case you were absent the day that they went over basic hydraulics in Physics 101, here is a brief update in reference to your BMW brake system; Fluids cannot be compressed, thus they are used as the medium to transmit force from a brake lever to the master cylinder (hydraulic force) continuing through the brake hoses to the brake calipers and finally to the brake disc and wheel. Pretty easy so far?

However, a property of brake fluid (Dot 4 is used in all BMW motorcycles) is that it is hydroscopic, meaning that it will over time absorb moisture (water) directly from the atmosphere (humidity). Moisture enters the braking system through microscopic pores in the rubber hoses, past the rubber seals on the brake caliper pistons and brake fluid reservoirs. The absorption of moisture is even more problematic in humid areas and/or where there is salt in the air. You can tell if moisture is in the brake fluid because it turns brown or even black from the nearly clear color when it is new. A deep brown / black brake fluid can contain as much as 8% water. In fact, Dot 4 brake fluid with only 3% water can loose as much as 50% of it braking power as measured in braking distance.

So what if it has some water? Well here is what happens; Brake fluid reaches high temperatures when riding in traffic or going downhill. The heat is generated by the friction of the brake system as well as engine heat. Water will boil / vaporize within the brake fluid if the fluid reaches the “boiling point”. The water vapor creates gas in the brake fluid system. When that happens, the brake lever will not be able to deliver the pressure necessary to compress the gas now in the brake lines (gasses unlike fluid can be compressed) to the level necessary to operate the brakes.

In addition, moisture within the brake fluid will, over time, cause corrosion and pitting in caliper pistons and bores, master cylinders, steel brake lines, and ABS modulators.

Note that brake fluids are not to be mixed. In addition, Dot 3 or 5 brake fluid is not approved for BMW motorcycles.

So what can you do? Simple, change the brake fluid approximatly every two years. Check your brake fluid, is it brown or clear? How long has it been since you had your brake fluid changed?

In addition, In cases where customers are requesting replacement of hydraulic brake components under the BMW Limited Warranty, BMW NA now requires verification of Service History to prove that the vehicles have been maintained properly.