What is the fluid in hydraulic bicycle brakes

5 mistakes in operating hydraulic disc brakes

In order to safely roll through life, you need a good reaction, observation and, most importantly, reference brakes.

fluid, hydraulic, bicycle, brakes

We have already told you what you need to know before choosing hydraulic disc brakes, which are considered the most in demand in terms of quality. And when you buy disc hydraulics for your iron horse, you should know what mistakes you should not make when using them (by the way, some of these principles can be applied to mechanical disc brakes).

So, a number of rules, following which, your rotors will last you longer and will require a little attention.

Never touch the braking surface of the rotor.

  • Firstly, after intensive braking, the rotor is very hot and you will definitely burn yourself.
  • Secondly, (KEEP ATTENTION!), Grease from the fingers can stain the rotor, which is fraught with a decrease in braking efficiency and the appearance of extraneous sounds during braking.

If oil accidentally gets on the rotor or pads, this will entail much more serious problems. If you notice that oil has dripped onto the rotor, for example, when inaccurately lubricating the chain with lubricant in aerosols or when pumping the hydraulic line), you need to carefully remove the wheel and wipe the rotor with a degreaser. At the same time, it is important that the oil does not get on the pads, because they have a spongy structure and willingly absorb oil and you cannot get rid of it with any degreaser. If oil does get on the pads, the brake will brake very badly, while making such sounds in which the pedestrians in front of whom you brake will rush into the loose without looking back. In this case, it is best to replace the pads.

True, there are a couple of “collective farm” methods that have a low percentage of success. If a little oil gets in, then you can try to grind the brake surface of the pads by removing a small layer of contaminated material, but a simpler and more effective method is to heat the pads. When exposed to high temperatures, oil trapped in the block will begin to burn out in the form of smoke or even a small flame. But here it is important not to overdo it. if the pads are heated too much, the braking surface can crumble.

Do not press the brake lever with the wheel removed. If you do this with hydraulic brakes without a rotor or spacer between the pads, the pistons will push until the pads close together and no longer go back on their own just enough to lock the rotor into place. Fortunately, this is easy to avoid: insert a special spacer instead of the rotor when removing the wheel. This spacer always comes with the brakes, but if you do not have one, then you can do with improvised means: use instead of the spacer, for example, an unnecessary plastic card or cardboard bent in half. But if your pads still “come together”, you can remove the pads and try to gently dilute them with a flat screwdriver. If you spread them more than necessary, then put the pads back in place, put the wheel and push the brake lever a little. the pads will take their place.

Remember to check the thickness of the brake pads. Although disc brakes have a long service life, the pads and rotor will still gradually wear out. If you see that the brake pad material is thinner than 2.5 mm, then it is time to change the pads. The rotor has outlived its life when its thickness becomes less than 1.5 mm.

Use the right tools and supplies if you intend to service your brakes yourself. Never confuse brake fluids. If your brakes use mineral oil, then in no case should you pump them with a pillbox and vice versa (which brake fluid is used in your brakes is usually indicated on the brake lever. see the picture).

If you make this mistake, you will fatally ruin your brakes. Do not line bent rotors with bare hands and you know why (remember the point). There is a special tool for this. Observe the correct tightening torques for the fasteners recommended by the manufacturer. To do this, you need a torque wrench.

Want to learn how to properly service your brakes? Consult our bike mechanics at:

  • m. Beresteyskaya, st. Degtyarevskaya, 51V 38-050-388-29-11
  • m. Poznyaki, st. Prince Zaton, 11 38-050-388-29-12

Or take the first course “Basics of Bicycle Mechanics” in our School of Bicycle Mechanics and service the braking system of your bike yourself.

Varieties of liquid based on working qualities

In this classification, it is appropriate to consider three main groups of hydraulic oils. The first is represented by basic formulations, characterized by balanced indicators of viscosity, compressibility and pressure. We can say that these are typical universal means of providing a liquid hydraulic function. The second group includes agents that are resistant to oxidation processes. This also includes thermally stable types of hydraulic fluids that can circulate under high pressure in contact with metal surfaces, water and air. The third group provides for a more perfect performance of the thermal protection function. These are compounds that are not subject to fire hazards even in close contact with fire sources.

Properties of oils for hydraulic systems

The quality of performance of the above functions is determined by the properties of a particular composition. The basic performance characteristics of hydraulic fluids include thermal shock resistance, viscosity, inertness and density. But special working qualities, including protective ones, are also of increasing importance. For example, anti-corrosion allows you to withstand liquids and damp environments without negative rusting processes. The hydraulic resistance of the liquid is also important, which determines the intensity of the working function of the composition. That is, the lower the resistance indicator, the easier the force is transferred from the power unit. As a result, less energy is spent to ensure the functioning of the installation. Another thing is that the achievement of optimal resistance indicators is rarely performed without losses in other technical and physical properties of hydraulic oils.

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Additives and fluid modifiers

It is often the additional components that play a decisive role in operational performance. They are mutually exclusive or complementary to each other, so it is impossible to get a completely universal tool suitable for any need. In varying degrees, the base base can be given such properties as anti-corrosion, aging resistance, extreme pressure and anti-wear properties.

In this case, the additives are divided according to the nature of their application. There are components that are added as an addition to the mineral base oil, and there are surfactants. For example, hydraulic brake fluid is obtained as a result of the inclusion of surface friction modifiers, which can be introduced into the composition already during the operation of the mechanism.

Basic oil additives are usually included in the factory. This category includes anti-foaming elements, antioxidants, etc. Active additives against this background will be beneficial in that they do not require special treatment of the liquid after adding.

Where are hydraulic fluids used??

Oils of this type are used in various technical devices and mechanisms. A classic example of their application is pipeline valves. By themselves, hydraulic devices are widely used in various fields of industry, manufacturing and construction. It can be press machines, units in factory lines, processing hydraulic systems, etc. It is important to note that hydraulic fluid can also be used in household equipment. Some models of pneumatic stations, pumping equipment and power units may also use these fluids. over, the functions of this type of oil are also different. they should be considered in more detail.

Hydraulic fluid formulations

The output product is usually concentrates based on technical oils and additives. A classic example is a product made with mineral oil and emulsifiers and diluted with anti-corrosion inhibitors. Actually, such a combination itself can act as a basis for the preparation of more technologically advanced modifications, which can also be combined with a huge range of elastomers. For example, to increase the hydraulic pressure of a fluid, manufacturers add seals to the formulations. Conversely, if it is necessary to achieve a higher degree of elasticity of the working component, emulsion lubricating oils are added.

Basic framework

As a base mineral oil, paraffinic formulations, naphthenic mixtures and various combined solutions can be used. There are also special modifications with improved basic working qualities. These are synthetic fluids, in the manufacture of which hydrocracking components, ester compounds and polyglycols are used, which are most often used for fire-resistant mixtures. The natural base bases from which biodegradable hydraulic oils are made are also used. Liquids of this type can contain plant-derived products that are environmentally friendly.

Regardless of the type of base oils, the quality of their refining is also important. There are different categories, differing in the degree of preliminary preparation of the composition. There are mixtures of coarse purification, and there are also oils that have passed multiple filtration. This is not to say that the second option will be the best in all use cases. In some areas, it is liquids that are based on a coarse elemental combination that manifest themselves optimally.

Fluid functions

The main task of a hydraulic fluid is to transmit pressure to the working component of the system. It can be a piston or a valve, the main thing is that the oil volume acts as a dynamic force transmitter and at the same time performs a number of auxiliary functions. For example, as already noted, technical oil provides lubrication of the rubbing elements of the working system, extending their service life. Depending on the operating conditions, it may also be necessary to perform special tasks.

For example, if the installation is planned to be operated in an environment subject to thermal effects or close contact with moisture, then the hydraulic fluid is replaced with a composition with suitable protective qualities. In this case, the technologist will recommend an oil with anti-corrosion properties and thermal resistance. At the same time, by default, every hydraulic fluid is designed to be cleaned. Pipelines are regularly flushed, as a result of which their inner surfaces get rid of sediments and other destructive substances.

How to choose a hydraulic fluid?

To a large extent, the choice of one or another composition is determined by the operating conditions. In particular, the operating temperature range, type of hydraulic system, pressure, environmental requirements and external influences should be considered. It is desirable to pay special attention to the viscosity index. If the task is to reduce leaks and increase sealing, then Honor mixtures with a minimum viscosity level should be preferred. The temperature of the working environment is also taken into account in a separate order. When deciding which hydraulic fluid to choose for a stationary system, you can give preference to compositions designed for a mode of 40-50 ° C. For mobile and dynamic systems, highly specialized fluid products are often selected.

Design and principle of operation

The principle of operation of mechanical and hydraulic disc brakes on a bicycle is basically the same. Pressing the brake lever sets the brake pads in motion and compresses the rotor. As a result, braking occurs due to the conversion of kinetic energy into heat due to the frictional force. The main difference lies in the method of transferring the pressing force of the brake lever to the brake pads. Mechanical disc brakes use a cable. When you press the handle, it is tensioned, which leads to displacement of the movable block.

In hydraulic disc brakes, its role is played by the piston system and hydraulic fluid. When the handle is pressed on the hydraulics, the piston of the master cylinder is set in motion. It pushes the hydraulic fluid that drives the pistons in the caliper, thereby compressing the pads.


But in terms of maintainability, disc mechanics wins, which can be relatively easily repaired in the field. In the event of a cable failure, it can be easily replaced on the spot, taking into account that you have a spare one. What can not be done with hydraulics. If you caught on a branch and damaged the hydraulic line, it is unlikely that you will succeed in installing a new one, filling in oil and pumping the system without special equipment. Plus, finding replacement parts for bicycle hydraulic disc brakes is much more difficult than for mechanical ones.

Another point where mechanics win is price. Good hydraulic brakes are more expensive than mechanical ones of the same level.

Bleeding the hydraulic system

Reasons for “blowing” the brakes:

  • large brake lever travel when pressed;
  • failure of the brake lever;
  • smooth movement of the handle, after the brake is applied.
  • take care of protecting discs and pads from oil ingress;
  • loosen the clamp of the brake lever.


  • Draw liquid into a syringe.
  • Expel any air bubbles.
  • Inject solution.
  • To prevent the valve from jumping out of the body, use a special sealing lining.
  • Unscrew the inlet nipple and supply fluid without dripping.
  • Tap the tubes to “expel” excess air from the main cylinder body.
  • Close the inlet nipple and remove the tube.
  • Screw on the plug.
  • Place the brake lever in a comfortable position.
  • Clean the steering system from oil residues with a special solution or a lint-free technical cloth.
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Cycling safety is directly related to the quality of the brakes. By choosing bicycles with hydraulic brakes, the user not only enjoys easy and quick braking while riding. He also takes care of himself and those around him.

Hydraulic brake for bike. device

The purpose of any brake mechanism is the same. to stop the vehicle.

  • hydroline;
  • brake rotor;
  • caliper;
  • brake lever.


Braking performance is independent of how the caliper and handle are linked. The material and size of the rubbing parts play a role here:

  • Rotor diameter.
  • Pads dimensions.
  • Friction lining material.

Usually, the pads are proportional to the overall diameters of the discs, of which only two are running:

  • 160 mm. light cross-country (forest paths, not long descents).
  • 185 mm. active driving on mountain roads.

Difference between disc hydraulics and mechanics

It is difficult for an inexperienced eye to determine which brakes are installed on a bicycle. hydraulic or mechanical, which is why it is unlikely that it will be possible to understand which is better and which is worse. Both there and there you will find a handle, a rotor and a brake caliper (the mechanism that presses the pads against the disc). And they have the same principle of operation. they clamped the handle on the steering wheel, the pads stopped the rotor.

The global difference lies in how they work, or rather, in the style of command transmission from the handle to the caliper:

The best hydraulic brakes for a bike

Bicycle companies are gradually switching to disc brakes. According to experts, they perform well, they work even in damp weather, they are durable, reliable, and, according to forecasts, in the near future they can displace other types of brake systems.

How to replace the brake fluid in hydraulic bicycle disc brakes. (Tutorial 002)

Pumping. the final stage of the repair

When the entire system has already been completely sorted out and assembled, it remains only to fill in the brake fluid and properly pump the hydraulics. The system is pumped in order to expel air bubbles from all pipes and cylinders, which prevent the hydraulics from working correctly.

There are a lot of ways how the hydraulic brakes of a bicycle are pumped. Anyone can do this at home, if they know how to handle tools at least a little. Someone uses syringes, and someone uses old and proven methods. Consider the simplest and most time-tested option.

To bleed the brakes, you will need the following components:

  • a transparent tube, suitable in diameter to the bleed bolt;
  • brake fluid or oil, depending on the specific system;
  • required keys;
  • jar or bottle;
  • a piece of clean cloth to keep it clean.

First you need to put a transparent hose on the bleeding bolt, and lower its other end into a jar, where there should be some brake fluid. Next, you need to open the brake fluid reservoir on the handle and make sure that it is at the maximum level. If necessary, add liquid.

Attention! The brake fluid is unique for each bike model and cannot be mixed or replaced.

After that, you can start pumping the disc brakes on your bike. Pressing the brake lever several times, you need to fix it in the pressed state and at the same time unscrew the screw for bleeding on the brake machine. In this case, you can observe how air bubbles will go out of the tube. After that, the screw must be tightened immediately, and only then you can release the handle and swing again.

This procedure must be repeated several times until the whole system works as it should. This can be felt by the stiffness of the brake lever.

Video showing the pumping procedure:

And one more way to pump the hydraulic line for the lazy:

Symptoms of a hydraulic brake malfunction

The first symptom of a malfunction of such bicycle brakes is that it begins to brake on its own. This can be explained by the fact that air has entered the brake system. This could have happened due to the fall of the bike, a low level of brake fluid in the reservoir, or when the hydraulic circuit is open.

Since air tends to be compressed, unlike liquids, when it enters the system, it works like a gas spring. That is, the air creates the pressure of the brake fluid, which activates the brakes.

Also, the braking system can engage in similar self-activity, in case of jamming of the working piston. And the reason for this is the ingress of water into the hydraulic system.

And, of course, the brakes urgently need to be repaired if the brake lever is not as elastic as before, or the hydraulics no longer responds to the cyclist’s command to stop the vehicle.

Troubleshooting and repair

To understand what exactly happened to the braking system of your faithful two-wheeled friend, you first need to do some experiments. The first step is to remove the wheel where the problem appeared. Next, you need to clean the brake machine well, the easiest way to do this is with a toothbrush. Well, the main goal is to remove the pads.

When access to the working pistons opens, you need to press them in with a screwdriver, and then gently press the brake lever. Both pistons should move forward. If one of them is stuck, you need to use a repair kit to eliminate the malfunction. Also, the piston system must be carefully inspected for leaks, if they are present, then the cylinder-piston group is worn out. As a rule, in this case, the pistons or special O-rings on them are changed.

Finally, you need to carefully inspect the entire hydraulic line. There should be no kinks, dents or other damage on it. If present, the hose must be changed.

Adjusting and bleeding hydraulic bike brakes

Hydraulic brakes are used primarily on bicycles where precision, responsiveness and reliability are required. First of all, these are heavy, massive mountain bikes designed to overcome steep slopes and climbs. Also, hydraulics have become an integral part of high-speed bicycles, as it allows you to very accurately and in the shortest possible time reduce speed.

The principle of operation of hydraulic brakes is very similar to mechanical brakes, where the brake pads are operated by a cable. The only difference is that in the hydraulics, instead of cables, brake fluid is used, and levers and eccentrics replace the cylinder-piston groups. Thanks to this approach, the frictional forces in the system are significantly reduced, and this increases its service life.

Despite this, hydraulics also has disadvantages, first of all, it is difficult to repair in case of breakdown. For example, in the event of a breakdown in the hydraulic line, the brakes cannot be repaired in the field. The most that is possible with the tools is to bleed the hydraulic brakes.

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Hydraulic brake device

Any hydraulic brake consists of at least two cylinders with pistons, connected by a line, or, more simply, a hose that can withstand high pressure. When the cyclist presses the brake lever, the piston displaces the brake fluid from the master cylinder and transfers it to the slave cylinder located in the machine. Here, under the action of pressure, the pistons begin to extend and press on the brake pads. And already due to the friction of the pads on the rotor (brake disc), braking occurs.

The cylinders in the brake machine are always larger than those in the brake lever. Due to this, in accordance with the law of hydraulics, the working brake pistons press on the pads several times more force than the cyclist does by pressing the handle. Also, this law works here due to the fact that two cylinders are installed on the brake machine, or even all 4, 2 on each side.

Repair Precautions

When repairing hydraulics on a bicycle, it is unacceptable for brake fluid to get on the pads or rotor. If this happens, the elements must be cleaned immediately with fine sandpaper. Otherwise, the brake pads will not work efficiently and quickly fail.

It is also important to know that brake fluid is poisonous and can be harmful to health, therefore it is not recommended to get it on the skin. Eye contact should be especially avoided.
In addition, it is a good solvent. it easily corrodes the bulk of paints and varnishes.

How to adjust everything yourself

It may be necessary to adjust the hydraulic brakes if you notice extraneous sounds while driving. creaking or rustling. A simple test will help to make sure that the adjustment is necessary: ​​lift the wheel of the bicycle and spin it. If the pads touch the brake disc, the wheel will quickly stop spinning.

The relative position of the pads and the rotor must be very accurate, because this affects the performance of the brake. To check the position of the disc in the typewriter and the position of the pads, place a white sheet under them and, if necessary, direct a bright light on it. against such a background it is easier to notice deviations.

The hydraulic brake is set as follows:
1. Loosen the caliper mounting bolts as much as possible so that it can move freely to the sides.

Squeeze the brake lever so that the pads are fully pressed against the rotor.

Check the position of the pad valves. they should be symmetrical, at the same distance from the rotor. Move the caliper slightly to the left or right if necessary.

Without releasing the brake lever, tighten the caliper mounting bolts.

Release the handle and check the brakes. Ideally, the pads should be at a minimum distance from the brake disc, but not touching it.

If you notice that one pad is located closer to the disc than the other, but the rotor itself is perfectly centered in the machine, do the following (preferably two people): press the disc against the more protruding pad and apply a little brake. Due to the fact that one valve works more than the other, they will align, and the problem will be eliminated.

The final touch is to set the position of the brake lever relative to the steering wheel. Sitting in the saddle, check if you feel comfortable using the brake lever and if you can quickly press it in an emergency. If the knob is too far for you, tighten the adjusting hex a little more. this will bring the knob closer, but at the same time the pads will move closer to the rotor. The main thing here is not to overdo it and not to make the stroke of the pads too small.

Video clearly showing the entire adjustment process:

Adjusting the bike’s hydraulic brakes and replacing the pads

The principle of operation of hydraulic brakes on a bicycle is to transfer pressure through the hydraulic line from the brake master cylinder to the brake cylinder in the caliper, which in turn moves the pads. The brake fluid is not compressed, therefore such a system is more reliable and efficient than a mechanical one.

On the other hand, hydraulic lines and the entire braking system require careful attention and constant maintenance, and therefore repairing and adjusting hydraulic brakes often seems to be a difficult task.

Replacing the brake pads

Poor operation of hydraulic disc brakes may be due to the following reasons:

  • damage or contamination of the hydraulic line or ingress of air into it;
  • complete abrasion of the brake pads;
  • dirty pads or disc.

In the first case, replacement of the hydraulic line or bleeding of the hydraulic brake is required, with the second and third cases it is easier to deal with.

First, take a close look at the brake pads. All manufacturers prescribe their minimum thickness, at which the braking performance does not decrease. By the way, the uneven wear of the left and right pads indicates that the rotor is not centered relative to the caliper.

As the pads wear out, the valves move closer to the rotor, so before installing new pads, you should slightly move them apart and re-adjust their position. The entire replacement procedure consists of the following sequence of steps:
1. Place the bike on a repair stand and remove the wheel.

Remove the retaining ring and unscrew the fixing bolt.

Pull out the old pads away from the wheel hub axle. Remember the location of the return spring between the pads. it releases them after the end of braking.

Thoroughly clean the surface of the valves and the inside of the caliper. Use a lint-free technical cloth and a mild cleaner such as isopropyl alcohol.

Using a plastic lever, such as a tube changer, press both valves into the caliper. To install the valve correctly, you need to press on its center, not on the edge.

Install a return spring between the new pads and insert them into the machine.

Tighten the fixing bolt and install the retaining ring.

Easiest Way to Add Fluid to MTB Hydraulic Disc Brakes | 4k

Clean the brake disc and replace the wheel. Check the correct position of the rotor, pads and the functionality of the entire system.

New pads may require a period of “adaptation”, so at first, the updated brakes should be used carefully.

it should be said that all work should be done with clean hands. Also, to keep the brakes less dirty, you should close them when lubricating the chain. With a comprehensive bike service, the brake system is repaired last.