Brake fluid.How it works and how often to change it

Today we will discuss the importance of the brake fluid. A rather neglected topic among drivers and I say this because every time I check a car in service with the brake fluid tester, I notice that it has over 4% concentration of water in the composition, a percentage that becomes dangerous for the braking system and which I will discuss below.

What is brake fluid good for?

The car’s braking system contains brake fluid to maintain its functional components. Like any mechanical system, the braking system needs a lubricating fluid, as its moving components tend to become warmer as they are used. If there is no brake fluid or if the brake fluid level is low, the braking system will not perform well and it will lead to serious braking problems.

Braking fluid, to the astonishment of all, has a limited life and overtime loses its properties. The brake fluid is made from petroleum-based substances, made in the laboratory, without water. Because it has to withstand huge temperatures without boiling, like the antifreeze.

But the brake fluid has a disadvantage: it attracts moisture. And that’s how it is tested for efficiency, with a device that measures the moisture in it. Brake fluid, by definition, is an incompressible liquid, not incompressible, and is based on Glycol, especially those DOT4, DOT3, or DOT 5.1.

What is brake fluid made of?

  • SOLVENT 60 TO 90%. Polyglycols have a high viscosity and therefore should be diluted with a low viscosity product (polyglycol ethers). Polyglycol ethers have the role of solvent in the liquid. They must dissolve the polyglycols and all the other components so that a single-phase liquid results for optimum lubrication at any temperature.
  • LUBRICATION BASE 5 UP TO 30. In brake fluids, polyglycols are used as lubricants in a proportion of up to 30%. They are obtained by the reaction between alkylene oxides (ethylene and propylene) and bifunctional components such as diols or water.
  • ADDITIVES 2 UP TO 5%. Usually, brake fluids contain up to 5% additives. Corrosion inhibitors are added to protect the metal elements of the braking system, and antioxidants to reduce the oxidation of polyglycol ethers and slow down the formation of residues and resins resulting from acid decomposition. These two types of inhibitors are essential to guarantee the optimum long-term quality of the brake fluid.

Characteristics of car brake fluid

  • Low compressibility. Brake fluids must maintain low compressibility and remain so even when temperature or pressure varies.
  • Constant boiling point. Water/humidity can be found in all braking systems. Moisture comes in various ways. For example, condensation can form on cables and stirrups. As the stirrups and cables are heated and scratched repeatedly, condensation occurs, after which moisture/water remains. Over time, moisture is trapped inside the stirrups and on the surface of the main cylinders. For the braking system to operate smoothly and safely, the brake fluid must maintain a certain viscosity at various temperatures, including extreme cold. This is especially important for systems equipped with ABS, traction control and stability control.
  • Lubrication of mobile components. Both the main cylinder and the secondary cylinders of disc or drum braking systems contain pistons that must move freely. To reduce friction and wear, add polyglycols to the brake fluids as lubricants.
  • Compatibility with materials. The brake fluids must be compatible with the materials of the braking system. Therefore, gasket expansion tests at 70 ° C and 120 ° C are performed on standard SBR and EPDM rubber samples, as well as on certain elastomers used by original equipment manufacturers (OEMs). During these tests, changes in volume, diameter, and hardness of the samples are measured.
  • Anti-corrosion protection. Additives (corrosion inhibitors) are added to the base liquid to protect against corrosion the metal components of the stirrups, the main cylinder, etc.
  • Resistance to oxidation and thermal stability. Braking is the conversion of kinetic energy into heat by friction. The amount of heat produced is considerable and depends on the weight and speed of the vehicle. However, under extreme operating conditions, the brake fluids reach temperatures above 150 ° C. The cracking phenomenon represents the measure of their thermal stability. This is determined according to the quality of glycol ethers and the quantity and nature of antioxidants. Antioxidants improve thermal stability and delay the aging of the liquid resulting from its oxidation into acidic components.

How does brake fluid work?

  • The driver presses the brake pedal;
  • The pedal, in turn, presses on a piston inside the central brake pump;
  • This action causes the pressure inside the brake cables to increase, which sets the brake fluid in motion;
  • Then, the pressure of the liquid causes the brake pads to press on the brake discs, being pushed by the pistons of the stirrups, plates that, later, make contact with the brake discs, slowing down and finally stopping their movement and the car.
Brake fluid reservoir

Types of brake fluid

While the functions of the brake fluid are somewhat easy to understand, there are a wide variety of types to choose from, which might be a little confusing when you decide to replace the brakes or brake fluid. The most important types of hydraulic fluid are those based on glycols – which are then classified into several classes – and those based on silicone.

Glycol-based brake fluid is usually used on cars with ABS systems, while those on silicone only work on cars without ABS systems.

If a car without an ABS has previously had glycol-based brake fluid, you cannot switch to a silicone-based brake fluid because small quantities of glycols residues in the wires can chemically compromise silicone integrity.

There are 4 types of brake fluid

  • DOT 3
    Brake fluids DOT 3 are usually glycol-based, but not because they should be. FMVSS116 does not mention the chemical composition of the brake fluids, but only imposes their physical properties. However, brake fluid manufacturers have agreed, as a rule, that glycol ether liquids are the cheapest way to meet the requirements.
  • DOT 4
    The DOT 4 brake fluids are also glycol-based but contain esters borate to improve some properties such as dry and wet boiling points. Brake fluids DOT 4 have a more stable and higher boiling point in the first part of their life, but ironically, as the liquid begins to absorb the water, its boiling point will drop faster than in the case of a regular DOT 3 liquid. According to FMVSS116 standards, DOT 4 brake fluids must have a minimum dry boiling point of 230 ° C and a minimum wet boiling point of 155 ° C.
  • DOT 5
    The DOT 5 brake fluids were designed for use in the military, that is, for vehicles that were stored for years without maintenance and needed to be operated whenever needed. In terms of their properties, they have a higher boiling point and corrosion/preservation, because they do not absorb water. Until now, car manufacturers have not opted to use silicone-based liquids on their cars for two reasons:
    They have a low solubility in the air, which creates the sponge sensation when pressing the brake pedal.
    They are not soluble in water, the moisture that enters the system can be corrosive and can freeze at low temperatures or boil at high temperatures.
  • DOT 5.1
    Historically, DOT 5 performance (especially boiling points and viscosity) could only be achieved with silicone-based liquids. However, modern blends have created glycolide-based liquids that meet DOT 5 requirements. As a result, DOT 5.1-level brake fluids have been created to differentiate the two different chemical compositions that meet the performance requirements for DOT brake fluids. 5. In short, DOT 5.1 brake fluids are DOT 4 type liquids that meet DOT 5 performance requirements. For this reason, they can be mixed with DOT 3 or DOT 4 fluids without any concern. Sometimes, they are also called “DOT 4 Plus” or “Super DOT 4” liquids because they look more like ordinary DOT 4 liquids in chemical composition than DOT 5 liquids. DOT 5.1 contains borate esters.

What does DOT mean to the brake fluid?

DOT means Department of Transportation, which is the body that regulates brake fluid manufacturers. The department ensures the quality and standard of the manufactured brake fluids.

Some useful tips about DOT at brake fluid:

  • – Do not use DOT 5 liquid for a car that needs DOT3 or DOT4;
  • – You can use DOT4 instead of DOT3, but not vice versa;
  • – Never mix the liquid in the car with another liquid;
  • – Do not spill brake fluid on body paint.

How do I know what DOT has the brake fluid I need to put in my car?

The safest method is to read the manual of the car or you can find it written on the lid of the brake fluid reservoir.

How often should you change the brake fluid?

Over time, the brake fluid can absorb moisture from the air, which can cause the fluid to degrade and stop working properly. Therefore, it is important to periodically remove the liquid completely and replace it with a new one, to ensure its proper functioning. Most mechanics suggest replacing the brake fluid every one or two years, although each car has its maintenance regime.

Since the brake fluid is essential for your safety, consult the car’s user manual to see how long it is recommended to change it, in your case.

As I said at the beginning of the article testing different brake fluids, I found that it has 4% water in it which is not good, so then it should be changed.

If it is still unclear how often you change the brake fluid or if you suspect something is wrong with the braking system, do not hesitate to contact a mechanic.

Symptoms of brake fuel leak are:

  • The brake warning light,
  • soft brake pedal,
  • the brake pedal is too far down,
  • visible fluid coming out,
  • and the worse one is that the car does not stop while braking.

How to add brake fluid?

  • first, find the liquid tank;
  • look in the tank, if it is at the “full” sign it means that you do not have to complete;
  • if it is below this sign, only fill with the original brake fluid, the same type of liquid that is currently in the tank;
  • if the brake fluid is under the “add” mark and is black, go to a mechanic. The brake pads or brake discs will need to be replaced and the oil replaced.

Transportation and storage of brake fluid

The brake fluid must be stored in the original container, in a clean and dry place, at room temperature or below it, preferably away from other petroleum products or liquid materials used for maintenance.

Always use the original packaging to make the product easy to recognize and close it tightly to avoid water absorption. In good storage conditions, new, sealed, brake fluid containers will have the following shelf life:

  • Container – shelf life 2 years
  • Can – covered, protected from light, stored on one side – shelf life 3 years
  • Bulk – stored in stainless steel barrels covered with a layer of nitrogen – shelf life 2 years

After opening, the content should be used as quickly as possible, within a few months.

What brake fluid to buy?

We recommend buying the following brake fluids from Amazon:

Bosch ESI6-32N Brake Fluid

Castrol 12509 Dot 4 Brake Fluid > best seler

ATE 706202 Original TYP 200 Racing Quality DOT 4 Brake Fluid

Motul Brake fluid, DOT 5.1


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