How does water get into the brake fluid? The brake fluid system is sealed, so how does it end up containing water?
Regarding this question, some say that water enters from the moisture in the air, while others say it happens when driving through water or during rainy conditions.
These explanations are not very clear. With the high pressure in the brake fluid system, if water could easily enter, then brake fluid would leak every time the brakes are applied. A more plausible explanation is that there is a small vent hole on the brake fluid cap, and brake fluid itself has good hygroscopic properties, so small amounts of water vapor can enter the brake fluid system through this vent hole.
When you apply the brakes, the high temperature generated by the friction between the brake pads and the rotor can be transferred to the brake fluid, raising its temperature to over 100 degrees Celsius. At this point, any water inside the fluid will turn into vapor, creating an empty space when the brake pedal is pressed. This is extremely dangerous. Therefore, when the brake fluid contains a high amount of water, it is recommended to replace it promptly.
Even if it exceeds the standard, you may not notice any difference. When using a water content tester, nearly 90% of vehicles, except for those within two years of purchase, will exceed the standard. However, you won't feel any difference when driving.
In such cases, there is no need to replace the brake fluid immediately. According to the maintenance manual, it can be replaced at the specified mileage intervals, such as 60,000 or 100,000 kilometers. If you frequently drive in rainy conditions or through water, as long as the brake fluid system doesn't have any leaks, there shouldn't be any issues.
However, due to the good hygroscopic properties of brake fluid, it is difficult to detect minor leaks on the brake master cylinder because water would wash away any traces. In this situation, if the brake pads are relatively worn and the brake fluid level is slightly lower, it is considered normal and may go unnoticed.
