How often and why should the brake fluid be changed. And is it necessary?
While servicing under warranty, you rarely thought about such an important safety component as brake fluid. But in vain. After all, it is she who makes the car brakes work and, without exaggeration, human lives depend on its quality and quantity.
How often do you need to change the "brake"? Is it possible to interfere with one "grade" of it with another? Is it necessary, if necessary, to top up or make a complete replacement? How to measure the degree of "wear" of the brake fluid? To understand these more than topical issues, let's first understand the concepts and technical details.
Brake fluid is a component of the brake system, with the help of which the force generated in the master brake cylinder is transmitted to the wheelsets.
For the proper functioning of brake mechanisms, the fluid must have a number of properties that are described in our country by the interstate standard. However, in practice it is customary to use the American quality standard FMVSS No. 116, which was developed by the US Department of Transportation (United States Department of Transport). It was he who gave birth to the abbreviation DOT, which has become a household name for brake fluid. This standard describes such characteristics as the degree of viscosity; boiling temperature; chemical inertness to materials (e.g. rubber); corrosion resistance; constancy of properties in the limit of operating temperatures; the possibility of lubrication of elements working in contact; the level of absorption of moisture from the surrounding atmosphere. In accordance with the FMVSS No. 116 standard, brake fluid mixture options are divided into five classes, each of which is designed for a specific type of work and even type of brake mechanisms - disc or drum.
MINERAL WITH CASTOR
The base for the brake fluid (up to 98%) are glycol compounds. Modern brake fluids based on them can include up to 10 or more separate components, which can be combined into 4 main groups: lubricating (polyethylene and polypropylene), which reduce friction in moving parts of brake mechanisms; solvent / diluent (glycol ether), on which the boiling point of the liquid and its viscosity depend; modifiers that prevent swelling of rubber seals and, finally, inhibitors that fight corrosion and oxidation.
There are also silicone-based brake fluids. Its advantages include such qualities as chemical inertness to most materials used in the construction of a car; wide range of operating temperatures - from -100 ° to + 350 ° С; invariability of viscosity at different temperatures; low hygroscopicity.
The mineral base in the form of a mixture of castor oil with various alcohols is currently unpopular due to its high viscosity and low boiling point. However, it provided an excellent degree of protection; low aggressiveness to paintwork; excellent lubricating properties and non-hygroscopic properties.
DANGEROUS DELUSION
Many people believe that the properties of the brake fluid do not change during operation, since it works in a confined space. This is a dangerous delusion. When you press the brake pedal, air enters the compensation holes in the system and the brake fluid absorbs moisture from it. The hygroscopicity of the “brake”, although it becomes a disadvantage over time, but it is necessary. This property allows you to get rid of drops of water in the brake system. Once in it, water can cause corrosion and freezing at low temperatures, which at worst leaves you without brakes in the winter, and at best leads to corrosion and costly repairs. But the more water is dissolved in the brake fluid, the lower its boiling point and the greater the viscosity at low temperatures. A brake fluid containing 3% water is enough to bring its boiling point down from 230°C to 165°C.
Exceeding the permissible percentage of moisture and lowering the boiling point can manifest itself in such a symptom as a single failure of the brake system and its return to correct operation. The symptom is very dangerous. It can indicate the formation of a vapor lock when the brake fluid with a high moisture content is heated excessively. Once the boiling brake fluid cools down again, the vapor condenses back into fluid and the car's braking performance is restored. This is called "invisible" brake failure - at first they do not work, and then "come to life". This is the cause of many unexplained accidents in which the inspector checks the brakes, not the brake fluid, and everything appears to be working properly.
The frequency of changing the brake fluid is indicated in the vehicle operating instructions and is usually from 1 to 3 years, depending on its type. The driving style should also be taken into account. If the driver makes frequent trips, it is not the time that should be counted, but the mileage. In this case, the maximum fluid life is 100 kilometers.
As Alexander Nikolaev, specialist of the TECHTSENTRIK service station, explains, “for most motorists it is recommended to use DOT4. This compound comes on all European cars from the manufacturer, while DOT5 is used for more aggressive driving. It absorbs water worse, which leads to corrosion. The average motorist should change the fluid every 60 km or every 000 years, racers change it before each race. Untimely replacement of the brake fluid will lead to the penetration of moisture, which entails the failure of the brake cylinders and caliper pistons. With an increased load, the heat transfer of the mechanisms is disturbed, which will cause the liquid to boil. The pedal will “get stuck” (with the highest probability this will happen in mountainous areas or on a serpentine), the brake discs will “lead” (deform), which will immediately manifest itself in a beating on the steering wheel into the pedal.
DEMAND NOT REPLENISHMENT, BUT REPLACEMENT
Another dangerous misconception is that the brake fluid can not be changed completely, but simply topped up as needed. In fact, it is necessary to regularly make a complete replacement of the brake fluid due to its, as already mentioned, hygroscopicity. Worn out brake fluid, when mixed with new fluid, will not achieve safety performance, which can lead to corrosion of the vehicle's interior, slower brake response to pedal input, and vapor lock.
BUT NOT MIX?
The easiest way to choose a brake fluid is to trust the brands. This is not such an expensive thing to save on it. Is it possible to add liquid, mix different brands? There is no single answer to this question. A number of experts believe that it is possible, but with the identity of the basic component, they recommend sticking to the products of one company. In order not to miss, it is worth remembering that solutions with silicone will have the inscription Silicone base (DOT 5 silicone base); mixtures with mineral components are designated as LHM; and formulations with polyglycols - Hydraulic DOT 5.
Bosch experts believe that brake fluid should be replaced, not only if it contains more than 3% moisture. Also, indications for a change are the repair of brake mechanisms or a long downtime of the machine. It is worth changing, of course, if you purchased your car from the aftermarket.
In addition to regular replacement, the decision to change the liquid can be made by assessing the degree of its "wear and tear" using technical means that determine the measurement of the boiling point and the percentage of water. The device - they are produced by many companies, in particular Bosch, is installed on the expansion tank of the hydraulic brake system and connected to the vehicle's battery. The measured boiling point is compared with the minimum allowable values for standards DOT3, DOT4, DOT5.1, on the basis of which a conclusion is made about the need to replace the fluid.
