Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years under non-racing conditions. Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Electronic testers and test strips are commercially available to measure moisture content, however, moisture test strips were taken off the market because they absorb moisture in the air before they can be used. The corrosion inhibitors also degrade over time. Degraded inhibitors cause corrosion in the braking system. The first metal to corrode is copper. You can determine when it is time to replace brake fluid when copper ions hit 200ppm. New fluid should always be stored in a sealed container to avoid moisture intrusion.
Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible — in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to compress the fluid’s chemical bonds. Most brake fluids used today are glycol-ether based, but mineral oil (Citroen/Rolls-Royce Liquide hydraulic mineral (LHM) and silicone (DOT 5) based fluids are also available.
Brake fluids must meet certain requirements as defined by various standards set by organizations such as the SAE, or local government equivalents. For example, most brake fluid sold in North America is classified by the US Department of Transportation (DOT) under its own ratings such as “DOT 3” and “DOT 4”. Their classifications broadly reflect the concerns addressed by the SAE’s specifications, but with local details – Alaska and the Azores have a different normal temperature and humidity ranges to consider, for example; many countries defer explicitly to the SAE specifications, or simply refer to “best practice” which in the application would defer to SAE standard. All approved fluids must be colorless or amber to be acceptable for street use in the U.S, except for DOT 5 silicone, which must be purple.
Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because the vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force – so the brakes will fail to stop the car.
Quality standards refer to a brake fluid’s “dry” and “wet” boiling points. Wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid’s boiling point after absorbing a certain amount of moisture. This is several (single digit) percents, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil-based formulations), are hydrophobic and can maintain an acceptable boiling point over the fluid’s service life.
The silicon-based fluid is more compressible than glycol-based fluid, leading to spongy feeling brakes. It can potentially suffer phase separation/water pooling and freezing/boiling in the system over time – the main reason single-phase hygroscopic fluids are used.
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