Brake fluids - what do you need to know about them?

Brake fluid has the function of transferring force (pressure) from the brake pedal to the brake pads. Chemically, it is a complex mixture that can contain a variety of components such as alkyl ethers, ethylene glycols, borate esters or polypropylene glycols. All brake fluids additionally contain different types of additives to improve their resistance to foaming and to improve other properties. These additives serve to improve the key performance characteristics of brake fluids, namely low viscosity and high boiling point.

What is the DOT?

On the brake fluid label, you will find DOT designations together with Arabic numerals, which indicate the intended use of the fluid. In the past, the DOT-3 generation was popular, but it is rarely used nowadays, mainly in vehicles from the 1980s and 1990s. Today, DOT-4 brake fluids are the most popular, commonly used in cars, motorbikes, trucks, buses and battery trucks. In some cars, you may find brake fluids labelled DOT-5, which contain silicones to improve moisture absorption and have a higher boiling point. There is also a DOT-5.1 category, mainly for vehicles equipped with ESP.

Characteristics of good brake fluid

Good quality brake fluid is characterised by:

  • Ensuring effective braking in all conditions, even during extreme situations.
  • Reducing the vehicle's braking response time when the brake pedal is pressed.
  • Protecting the brake system from corrosion and leaks from rubber hoses and seals.
  • Possessing optimum hygroscopic properties, allowing it to absorb water, avoiding it freezing in the lines in winter and causing the fluid's minimum boiling point to increase.
  • Extending the life of the brake system and extending the life of the vehicle, contributing to its overall good condition.

In short: by selecting good quality brake fluid, we are guaranteed higher driving safety and faster braking.

Poor quality brake fluids and their consequences for the car

Out-of-date or inadequate brake fluid can lead to significantly increased stopping distances and pose the risk of the brakes not working at all. Brake system failure can occur when least expected, putting the driver in a dangerous situation.

Brake fluid and its composition

This fluid is a composition of glycol (alkyl ethers of alkylene glycols) along with a number of additives that serve different purposes. Brake fluid also includes lubricants such as ethylene polyglycols, propylene polyglycols and boron esters. The function of additives in brake fluids, on the other hand, is performed by antioxidants, anti-corrosives, stabilisers and other additives that improve the overall properties of the fluid. Typically, brake fluid contains approximately: 70-80% solvent (glycol), 20-30% lubricant, 1 to a few percent other additives.

Can operating fluids be mixed together?

Conventional glycol brake fluids, such as DOT 3, DOT 4, DOT 5.1 and DOT-4 LV, are compatible with each other and can be mixed. In case of emergencies, for example when travelling, it is acceptable to use the higher grade fluid as a substitute for the lower grade DOT fluid.

What liquids cannot be mixed?

However, please note that DOT 5 fluid, which is silicone-based, cannot be mixed with other types of fluid. DOT 5 fluid is typically used in transport vehicles for military applications, which are often left unused for long periods of time and require immediate efficiency. This type of fluid does not absorb moisture, which ensures a stable boiling point over a long period of time.

Another type not miscible with DOT fluids is the LHM type fluid, which is required in older French cars. LHM is a mineral oil-based hydraulic fluid that does not absorb moisture and has a high viscosity at high temperatures.

Can engine oil be mixed?

Miscibility means that you can add oil without fear, as there will be no adverse effects when mixing two different oils, with no problems with the functioning of the lubrication system.

Most modern engines require synthetic fluids to be used in them. The optimal solution is to use exactly the same oil that is already present in the drive unit, but these operating fluids are compatible with each other and can be mixed. If the engine oil from another manufacturer has the same quality grade and preferably also the viscosity, there is no obstacle to adding it to the engine in an emergency situation.

Mixing of coolants

Operating fluids for radiators come in different types. The differences between them are due to different sets of additives, for example anti-corrosive additives, which can interact in undesirable ways if mixed together. For example, combining IAT fluid with OAT fluid can result in precipitates that can clog the radiator ports, reducing radiator performance. In an emergency, however, coolants of the same type but from different manufacturers can be mixed. However, it is not advisable to be guided by colour, as this has little to do with the properties of the coolant.

Mixing of gear oils

These fluids have a low viscosity and an adequate coefficient of friction. We should not mix them with oils of a different viscosity, which have different parameters. This can lead to damage to the control system, its seizure or overheating of the transmission.

The development of the automotive industry and the increasing importance of road safety are forcing manufacturers to create high-quality brake systems and brake fluid. Nowadays, most cars are equipped with hydraulic systems that are filled with this substance. The functioning of these systems has a direct impact on the safety of users on the road, so attention to their efficiency is extremely important. Regular replacement of brake fluid with new fluid ensures comfortable and safe driving.

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