Brake Fluid (BF) is the most important consumable component of the braking system, and its replacement is an essential operation to ensure the proper operation of the car and the safety of the people who use it. Each car’s owner’s manual will tell you when to change the brake fluid. According to some experts, the brake fluid should be changed at least once every three years, others – once every two years (after 30 000-60 000 km of run).
What is brake fluid and how it works
There are factors according to which the brake fluid needs to be changed more often. For example, if you have a sports car, you need to change the fluid after every 15,000 km. The category of fluid, driving style, load on the car, operating conditions, and the quality of the road surface also matter. Changing brake fluid has its own subtleties.
Brake fluid is an auto chemical product. When you press the brake pedal, a powerful force is transmitted to the brake calipers or cylinders, which slow down the rotation of the car wheels. This process is accompanied by the release of thermal energy.
The task of the brake fluid is to prevent boiling and do it well, quickly and repeatedly. Special additives in the brake fluid are responsible for this function.
The signs of a good brake fluid:
- does not boil under heavy braking;
- does not freeze under severe frost;
- has a low viscosity, which allows you to quickly transfer the force on the hoses;
- does not absorb moisture and does not allow its accumulation in braking mechanisms;
- is inert with respect to rubber or plastic hoses and gaskets of brake system.
Basic signs of the necessity to change the brake fluid
Fresh brake fluid boils at temperatures between plus 200 and plus 260 degrees. But since it does absorb moisture, over time, when the water content exceeds 3% of the total volume of the fluid, this threshold drops to values from plus 150 to plus 180 degrees. When the boiling point is reached, air bubbles appear. They cause the brake fluid to be partially discharged into the reserve tank, while lowering the level in the main tank.
The same happens if the system is depressurized (even one circuit) and air enters it. As a result, during braking there is not enough pressure, and the brake pedal collapses. And it is fraught with an accident!
In addition, the additives in the brake fluid tend to “age” over time. They decompose and lose their original characteristics.
So, the replacement of the brake fluid is necessary if the following symptoms are present:
- abnormal operation of the brake pedal;
- change of fluid color;
- appearance of turbidity, stratification or sediment;
- Violation of the percentage of moisture – this can be checked at the service station or with the help of special testers;
- Depressurization of the brake system.
The quality of the brake fluid directly affects the performance of the brake system. In a difficult road situation the brakes have no right to fail!
Brake fluid standards
The brake systems of modern cars use fluid that meets the DOT 4 standard. It includes:
- polyglycol esters;
- anti-rust, anti-oxidation, plasticizer, anti-foam additives
- boric acid to reduce hygroscopic properties.
Boiling point of basic DOT 4 in laboratory conditions is from plus 230 degrees, and in practice – from plus 155 degrees, its viscosity – 1800 mm2/s. This standard requires that the fluid be replaced every two years. DOT 4 Class 6 boils at temperatures from plus 175 degrees in standard vehicle operation.
There are other standards:
- DOT 2 – no longer in use;
- DOT 3 – boils at temperature plus 205 degrees (new), at temperature plus 140 degrees (old), viscosity is 1500 mm2/s, it is applied only on old cars, for this standard the period of replacement of the FAME is provided once in 1,5 years;
- DOT 5 – this fluid is based on silicones instead of glycols, so it is not compatible with other standards, this standard requires changing the fluid every 4-5 years;
- DOT 5.1 – boils at temperature plus 260 degrees (new), at temperature plus 180 degrees (old), it is used in cars with high-speed ABS, its boiling point in “wetted state” is plus 185 degrees, kinetic viscosity – 900 mm2/s, that is enough low.
The lower the viscosity of the brake fluid, the freer and faster it transmits the force to the actuators of the brake system. At the same time the higher the boiling point of the fluid, the lower the probability of “brake failure” on the road, because it is impossible to transmit the necessary force to the pads through the vapor.
Some auto chemical manufacturers have their own products in their product lines. For example, Racing Brake Fluid from the German company Liqui Moli boils at a temperature of plus 195 degrees. This product is designed for sports and rally cars, in which the braking system is under considerable stress.
How to change the brake fluid
Ideally, when the brake fluid fails, you need to drain it completely and pour in a new one, and then pump the system. Dumping is not recommended by either manufacturers or car owners themselves, as mixing the compositions may adversely affect the operation of the brake system.
But still some car owners use this method – remove some of the old brake fluid with a syringe. They leave a little bit of it to prevent air from getting into the system. Then with the help of a transparent tube the rest of the fluid is poured into some vessel. And finally fill the tank completely with the new brake fluid.