The compression of a vapor lock when applying the brake pedal can lead to total hydraulic brake system failure. After several years of service, it is not unusual to find brake fluid that contains as much as seven to eight percent water.Įxcessive water content will decrease the boiling point of brake fluid and increase the risk of vapor lock. After 18 months, the level of contamination can be as high as three percent. After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. Degraded inhibitors in conjunction with the presence of excessive water cause corrosion in the braking system. The corrosion inhibitors in brake fluid degrade over time. Electronic testers and test strips are commercially available to measure both moisture and copper content. As compressibility increases, more brake pedal travel is necessary for the same amount of brake caliper piston and wheel cylinder force, and the brake pedal feels different.īrake fluid should be tested for water and copper content. Whether you have been driving and using the brakes all day, or you have just started your vehicle and used the brakes for the first time, the pedal should feel the same. This is important to ensure consistent brake pedal feel. Additives (corrosion inhibitors) are added to the base brake fluid to help accomplish this.īrake fluids must maintain a low level of compressibility, even with varying temperatures, to accommodate different environmental conditions. The fluid must also protect against corrosion as moisture enters the system. Brake fluid with the wrong viscosity can have fatal consequences for the function of modern brake systems.īrake fluid is designed to protect the metals used inside components such as calipers, wheel cylinders, master cylinders, and ABS control valves. The hydraulic units in these systems have numerous small holes and channels, some of which are smaller than the diameter of a human hair. This is especially important in vehicles equipped with an anti-lock braking system (ABS), traction control, and stability control (ESP).
Viscosity can be thought of as the thickness of fluid, or the resistance to flow. The problem is obviously worse in wet climates where humidity is high.įor reliable and consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme hot and cold. The fluid attracts moisture through microscopic pores in rubber hoses, past seals, and exposure to the air. Glycol based brake fluid starts to absorb moisture from the moment it is put in the hydraulic brake system or exposed to the air. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic, which means they absorb moisture from the atmosphere. Most brake fluids used today are glycol-ether based. Vaporization is a problem because vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force which will result in the brakes failing to stop the vehicle. It must have a high boiling point to avoid vaporizing within the lines.
Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It works because liquids are incompressible.īrake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly. Simply stated, when you apply your foot to the brake pedal, brake fluid transfers this force into pressure to the front and rear brakes and stops the vehicle. It is used to transfer force into pressure, and to amplify braking force. Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in vehicles.
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