Brake fluid is a type of hydraulic fluid used in hydraulic brake applications in motorcycles, automobiles, light trucks, and some advanced bicycles. It is used to transfer force under pressure from where it is created through hydraulic lines to the braking mechanism near the wheels. It works because liquids are not appreciably compressible – in their natural state the component molecules don’t have internal voids and the molecules pack together well, so bulk forces are directly transferred to trying to compress the fluid’s chemical bonds.
Brake fluid is also commonly used in hydraulic clutches.
Most brake fluids used today are glycol-ether based, but mineral oil (Citroën liquide hydraulique minéral 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’s Department of Transportation (DOT) under their 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 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 practice would defer to the SAE.
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.
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 vapor released into the lines is compressible and would result in an inability of the hydraulic fluid to transfer braking force. Quality standards refer to a brake fluid’s “dry” and “wet” boiling points. Wet boiling point, which is usually much lower, refers to the fluid’s boiling point after absorbing a certain amount of moisture. This is several percent, varying from formulation to formulation; in higher levels, the moisture itself can boil separately from the base fluid. Glycol-ether/dot three/dot four brake fluids are hygroscopic (water loving), which means they absorb moisture from the atmosphere under normal humidity levels. More modern fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point as they absorb moisture over the fluid’s service life.
For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme cold. This is especially important in systems with Antilock brakes (ABS), Traction Control, and Stability Control.
Brakes fluids must not corrode the metals used inside components such as calipers, master cylinders, etc. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this.
Brake fluids must maintain low level of compressibility that remains low, even with varying temperatures.
Service and maintenance
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. 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. The corrosion inhibitors also degrade over time. New fluid should always be stored in a sealed container to avoid moisture intrusion. DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should be used only to fill non-ABS systems that haven’t been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture, glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid doesn’t allow moisture to enter the system, but doesn’t disperse any that is already there either. A system filled from dry with silicone fluid, does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s.
Brake fluid is not considered a “top up” fluid. If it is low, there is usually a problem. Brake fluid level in the master cylinder will drop as the linings (pads or shoes) wear and the calipers or wheel cylinders extend further to compensate. This added fluid may need to be removed when renewing pads or shoes. Overspill from pushing back pistons should be avoided, because glycol (but not silicone) fluid, will lift or strip paints and other coatings on contact. Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently, a significant loss of braking ability. Modern cars have split hydraulic circuits to ensure against total hydraulic failure. As a general rule, brake fluids with different DOT ratings should not be mixed, although all DOT fluid is compatible, (this is part of the DOT specification). This is because it will dilute and reduce the properties of the higher specification DOT fluid, or in the case of mixing of glycol with silicone fluid may cause corrosion due to trapped moisture.
Brake fluid can be dangerous as it is toxic and highly flammable.