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, since it allows you to drop speed very accurately and in the shortest possible time.
The principle of operation of hydraulic brakes is very similar to mechanical brakes, where the brake pads are actuated 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, if the hydraulic line breaks down, the brakes will not work in the field. The most that is possible with the tools is to bleed the hydraulic brakes.
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 influence 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.
Hydraulic system diagram
The cylinders in the brake machine are always larger than those in the brake lever. Thanks 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.
Symptoms of a hydraulic brake malfunction
The first symptom of a malfunctioning bike brake is that it starts to slow down on its own. This can be explained by the fact that air has entered the brake system. This could be due to a fall of the bike, a low level of brake fluid in the reservoir, or an opening of the hydraulic circuit.
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 sealing rings on them are replaced.
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 replaced.
Pumping - the final stage of repair
When the entire system has already been completely sorted out and assembled, it remains only to fill in the brake fluid and correctly 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. Some use syringes, while others use 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, the liquid must be topped up.
Attention! The brake fluid is unique to each bike model and cannot be mixed or replaced.
After that, you can start bleeding 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 it 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 hose for the lazy:
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 will 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. In particular, contact with the eyes should be avoided.
In addition, it is a good solvent - it easily corrodes the bulk of paints and varnishes.
Hydraulic brakes on a bicycle provide predictable responsiveness, absolute accuracy, and are much more powerful than mechanical brakes, so they are mainly installed for extreme and high-speed riding.
The mechanism of action of the hydraulics is similar to the mechanical one: the brakes begin to work as a result of the tension of the cables, but in the hydraulic system, instead of cables, brake fluid works, and the lever and eccentric are replaced by a cylinder-piston group.
Therefore, hydraulic brakes are much easier to brake, because braking requires less effort than mechanical brakes.
But, unlike mechanics, hydraulic brakes are much more difficult to repair, if the hydraulic line breaks down, it is impossible to repair the system in field conditions, the only thing that can be done without professional equipment is to bleed the brake system.
You can read about what bicycle brakes are.
Hydraulic structure
The hydraulic brake consists of a "reservoir" with fluid on the brake lever, the hydraulic line itself and a caliper consisting of a cylinder and a piston.
The braking response begins as a result of pressing the brake lever, which drives the piston, which in turn squeezes out the fluid from the main reservoir and directs it along the hydraulic line to the working area.
In the cylinder, under the pressure of the fluid, the pistons come into motion and act on the pads, as a result of friction, braking occurs.
The cylindrical mechanism in the brake machine is always larger in size than in the control lever, therefore the pressure on the brake pads is produced in a reinforced size, much higher than the pressure on the lever.
Bicycles, scooters, accessories
Also, the load increases when multiple brake cylinders are installed.
Breakdown of the mechanism
The main symptom of a hydraulic breakdown is the "failure" of the brake lever during braking.
This is due to the appearance of air bubbles in the hydraulic system, due to a fall, a decrease in the level of the liquid or a rupture of the connected circuit inside the hydraulics.
When air enters, it is compressed, creates pressure, sets the pistons in motion and starts the mechanism.
To find out the cause of the failure, it is necessary to exclude the usual pollution, for this, the pads are also removed, the brake machine is cleaned.
After that, the pistons are pressed in with a special device: the brake lever is pressed until both pistons are fully extended, if they are jammed, the cylindrical system is worn out, in this case the pistons and special sealing rings are changed, the oil in the system is replaced.
Also, spontaneous brake operation can occur as a result of seizure of the brake piston after water ingress.
The hydraulics must be repaired even in the event of minor failures.
In addition to mechanical damage to the shell, over time, brake fluid or hydraulic oil changes its consistency and begins to absorb air and moisture through microscopic gaps.
As a result, the fluid changes its color, the brake lever collapses, and the efficiency of the system decreases.
Air infiltration is also possible due to the expansion of the tank, this happens when you pull the lever on an inverted bike.
To resume normal operation, the hydraulic brakes on the bike must be serviced: completely bleed the hydraulic system.
Blowing hydraulics
Hydraulic fluid varies from manufacturer to manufacturer.
In the hydraulic system of bicycles from Shimanu, Tektru, Maguru - mineral or semi-synthetic oils, all other companies use DOT brake fluid.
It should also be borne in mind that Avit and Formulu hydraulics do not have connecting pipes for pumping, so you need a syringe set with an M5 / 0.8 sleeve.
The main difference between fluids: brake fluid DOT is hygroscopic, that is, over time it absorbs moisture and may lose its properties, it should be changed every 2 years, regardless of mileage, oils do not absorb moisture, but over time it darkens, and if it is still water gets into the liquid, then when mixed it becomes "whitish".
In addition, mineral oils are not chemically aggressive and do not harm the plastic or paintwork of the bike.
Service methods
There are two ways to service a hydraulic bicycle brake:
Direct pumping
With direct pumping of the hydraulic system, oil is poured directly into the expansion tank and, after clamping the lever, is directed down the hydraulic system.
During operation, it is necessary to constantly monitor the oil level indicator and add a new portion of fluid in order to prevent the tank from emptying, while tapping with a key or a screwdriver on the reservoir and the hydraulic line in order to expel air from the system.
During the passage of the liquid, the hose is closed, after which the lever is lowered several times to the limit and the valve is opened. Under the influence of pressure, air flows into the pipe, the brake lever is held and the valve is closed.
Liquid is poured into the expansion tank and this continues until the appearance of an oil of a uniform consistency and without air bubbles.
At the end of the operation, brake fluid is added and the tank is closed.
Reverse pumping
- A syringe with a volume of 200 ml is introduced through a short tube to the valve of the caliper;
- The lever closes and air is sucked in from the caliper and hydraulic cord;
- The valve of the caliper closes, the hose with the syringe is disconnected, air bubbles are squeezed out;
- The syringe is inserted into place and the procedure is repeated until the hydraulics are completely free of air;
- The next step is to completely fill the hydraulic system with brake fluid.
In this way, it is convenient to pump fluid into the brakes if it is not possible to squeeze out all the air from the system during direct pumping. And pumping in the first way takes more time.
Also, in this way, air is pumped out from the opposite half of the caliper without its own valve.
Detailed video guide to servicing hydraulic brakes:
One of the fluids important for the normal operation of a car is brake fluids. About what this fluid is needed for, how often it needs to be replaced and what kind of brake fluids to use for optimal operation of the machine's braking system - in our today's article.
The role of brake fluid in the "body" of a car
The braking system, which is responsible for the timely stop of the car and therefore plays an important role for the safety of the passengers of the car, cannot work without brake fluid (TK). It is she who performs the main function of the brake system - it transfers through the hydraulic drive the force from pressing the brake pedal to the brake mechanisms of the wheels - pads and discs, as a result of which the car stops. Therefore, even in driving schools, novice motorists are strongly advised to periodically check the levels of four service fluids: glass cleaner and brake fluid, on which the optimal operation of the car depends.
Composition and properties of brake fluids
The basis of the chemical composition of most brake fluids is polyglycol (up to 98%), less often manufacturers use silicone (up to 93%). In the brake fluids used on Soviet cars, the base was mineral (castor oil with alcohol in a 1: 1 ratio). It is not recommended to use such fluids in modern cars because of their increased kinetic viscosity (thicken at -20 °) and low boiling point (at least 150 °).
The remaining percentages in polyglycol and silicone TK are represented by various additives that improve the characteristics of the brake fluid base and perform a number of useful functions, such as protecting the surfaces of the working mechanisms of the brake system or preventing the oxidation of TK as a result of exposure to high temperatures.
It is not for nothing that we dwelt in detail on the chemical composition of brake fluids used in cars, since many motorists are interested in the question - "is it possible to mix technical specifications with different chemical bases?" We answer: Mineral brake fluids are strongly not recommended to be mixed with polyglycolic and silicone fluids. From the interaction of the mineral and synthetic bases of these fluids, clots of castor oil can form, which clog the lines of the brake system, and this is fraught with malfunctions of the brake system. If you mix mineral and polyglycolic TK, then this "hellish mixture" will be absorbed into the surface of the rubber cuffs of the brake hydraulic drive parts, which will lead to their swelling and loss of sealing.
Polyglycolic TZ, although they have a similar chemical composition, and can be interchangeable and, but mixing them in one brake system is still not recommended. The fact is that each manufacturer of technical specifications can change the composition of the additives at its discretion, and their mixing can lead to a deterioration in the main operational characteristics of the working fluid - viscosity, boiling point, hygroscopicity (ability to absorb water) or lubricating properties.
Silicone brake fluids it is forbidden to mix with mineral and polyglycolic ones, as as a result, the working environment is clogged with precipitated chemical substances, which will lead to clogging of the brake system lines and failure of the brake cylinder assemblies.
Classification of brake fluids
Today, most countries in the world have uniform brake fluid standards known as DOT (after the name of the agency that developed them - the Department of Transportation - the United States Department of Transportation) - such markings can often be found on brake fluid containers. It means that the product is manufactured in accordance with the regulatory federal vehicle safety standards FMVSS No. 116 and can be used in the braking systems of cars and trucks, depending on the technical characteristics of these vehicles. In addition to the American standard, brake fluids are labeled in accordance with the standards adopted in a number of European and Asian countries (ISO 4925, SAE J 1703 and others).
But they all classify brake fluids according to two parameters - their kinematic viscosity and boiling point. The first is responsible for the ability of the working fluid to circulate in the brake system line (hydraulic drive, pipes) at extreme operating temperatures: from -40 to +100 degrees Celsius. The second is to prevent the formation of a vapor "plug" that forms at high temperatures and can lead to the brake pedal not being triggered at the right time. When classifying TZ by boiling point, two states are distinguished - the boiling point of a liquid without impurities of water ("dry" TZ) and the boiling point of a liquid containing up to 3.5% water ("humidified" TZ). The "dry" boiling point of the brake fluid is determined by a new, freshly poured working fluid, which did not have time to "collect" water and therefore has high performance characteristics. The “humidified” boiling point of TK refers to a working fluid that has been in operation for 2-3 years and contains a certain amount of moisture in its composition. More about this - in the section "Service life of brake fluids". Depending on these parameters, all brake fluids are divided into four classes.
DOT 3. The "dry" boiling point of this brake fluid is not less than 205 °, and the “wet” one is not less than 140 °. The kinematic viscosity of such a TZ at + 100 ° is not more than 1.5 mm² / s, and at -40 - not less than 1500 mm² / s. The color of this brake fluid is light yellow. Application: intended for use in cars, the maximum speed of which is not more than 160 km / h, in the braking system of which disc (on the front axle) and drum (on the rear axle) brakes are used.
DOT-3
DOT 4. The "dry" boiling point of this brake fluid is not less than 230 °, and the "wet" one is not less than 155 °. The kinematic viscosity of such a TZ at + 100 ° is not more than 1.5 mm² / s, and at -40 - not less than 1800 mm² / s. The color of this brake fluid is yellow. Application: intended for use in vehicles with a maximum speed of up to 220 km / h. The braking system of such vehicles is equipped with disc (ventilated) brakes.
DOT 5. The "dry" boiling point of this brake fluid is not less than 260 °, and the "wet" one is not less than 180 °. The kinematic viscosity of such TZ at + 100 ° is not more than 1.5 mm² / s, and at -40 - not less than 900 mm² / s. The color of this brake fluid is dark red. In contrast to the above TK, DOT 5 is based on silicone, not polyglycol. Application: intended for use on special vehicles operating in conditions of extreme temperatures for braking systems, and therefore is not used on ordinary cars.
The "dry" boiling point of this brake fluid is not less than 270 °, and the “wet” one is not less than 190 °. The kinematic viscosity of such TZ at + 100 ° is not more than 1.5 mm² / s, and at -40 - not less than 900 mm² / s. The color of this brake fluid is light brown. Application: intended for use in braking systems of sports racing cars in which the temperatures of working fluids reach critical values.
Pros and cons of brake fluids
All of the above brake fluids have their own advantages and disadvantages. For convenience, we will indicate them in the table below:
| TK class | Dignity | disadvantages |
| DOT 3 |
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| DOT 4 |
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| DOT 5 |
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| DOT 5 .1 |
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When to change the brake fluid?
The service life of the brake fluid directly depends on its chemical composition.
Mineral TK, due to its chemical characteristics (low hygroscopicity, good lubricating properties), has a rather long service life (up to 10 years). But when water enters the liquid, for example, in the event of a depressurization of the brake system, its properties change (the boiling point drops, the viscosity rises), and it can no longer perform its functions, which can lead to brake failure. Periodic inspection (once a year) of the brake system and fluid condition, which can be determined in laboratory conditions, is recommended.
Polyglycolic TK has an average or high degree of hygroscopicity, and therefore its condition should be checked twice a year. It is possible to assess the state of polyglycolic TK visually: if the liquid has darkened or precipitation is noticeable in it, then it must be completely replaced. Such TK is capable of absorbing up to 3% moisture per year. If this figure exceeds 8%, then the boiling point of the brake fluid can drop to 100 °, which will lead to boiling of the TK and failure of the entire brake system. Automotive manufacturers recommend changing polyglycol-based brake fluid every 40 thousand kilometers or every 2-3 years. Typically, this brake fluid is completely changed during the installation of new external braking mechanisms (pads and discs).
Silicone TK is distinguished by its durability of operation, since its chemical composition is more resistant to external influences (moisture ingress). As a rule, replacement of silicone brake fluids is carried out after 10-15 years from the moment of filling the brake system.
The design of the brakes of different manufacturers is very different from each other, therefore the recommendations for the use of brake fluid are also very different.
Before servicing your bike's brakes, be sure to read the manufacturer's instructions!
In particular, the differences relate to the recommended brake fluid. For example, Shimano produces a special mineral oil for its brakes, and only this oil can be used in all models of hydraulic brakes from Shimano. Hope recommends using DOT 4 or DOT 5.1 automotive brake fluid.
There are quite stringent requirements for brake fluid:
- It must not corrode the metals of which the brake parts are made, nor must it destroy oil seals and seals.
- It should not thicken in the cold.
- It should not expand much when heated (and the disc brake caliper can heat up to very high temperatures during prolonged braking).
- It should not boil when heated (brake failure on long descents is most often associated with heating brake parts and subsequent boiling of the brake fluid)
- It must be able to chemically bind the water that gets into it (water in the hydraulic system of the brakes not only causes corrosion, but can also boil when the caliper heats up).
Sooner or later, water will enter the hydraulic system anyway, and the ability of the brake fluid to bind this water is limited. Therefore, the brake fluid must be replaced periodically. Usually this does not have to be done often - once every few years.
Changing the brake fluid is described using the example of Shimano 525 brakes.
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The SHIMANO mineral oil has a bright red color, which fades and becomes discolored over time. The brake fluid should be changed when it loses its color, becomes pale pink .. |
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Checking the condition of the brake fluid 1. Loosen the screws securing the brake lever to the steering tube. |
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3. Unscrew the two screws and remove the cover from the expansion tank. |
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We look at the color of the liquid in the expansion tank. If it is red (as in the photo), then close the expansion tank and put the brake lever in place. If the liquid in the tank is colorless, or has a pale pink color, then it is time to change it. |
Replacing the brake fluid
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For replacement, you will need, in addition to screwdrivers and keys, a piece of PVC tube 30-40 cm long (preferably translucent or transparent) and a basin for waste liquid. It is convenient to pour brake fluid into the expansion tank from a medical syringe. The best way to change the brake fluid is to remove the brake caliper from the bike. In this case, there is no need to be afraid of the brake fluid getting on the disc and pads. In addition, the brakes are easier to bleed when the brake lines are vertical. Place some kind of solid spacer between the brake pads (a piece of cardboard or plastic the same thickness as the brake disc) |
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| We drain the old brake fluid.
1. Put a tube on the valve located on the caliper, and direct the other end of the tube into the basin. 2. Open the valve with a key. |
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3. Press the brake lever several times and observe how the old brake fluid is poured into the bowl from the tube. 4. When the old fluid stops pouring out, proceed to filling the hydraulic system. |
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Fill in new brake fluid and bleed the brakes. Let us check that |
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| 1. Pour the brake fluid into the expansion tank to the brim. (You can use a medical syringe) 2. Press the brake lever several times. At the same time, air bubbles rise into the expansion tank, and the level of the brake fluid in the tank decreases - it passes into the hydraulic line. As the fluid level in the reservoir decreases, it is necessary to add new brake fluid there, preventing the reservoir from completely emptying. To make air bubbles rise up to the expansion tank, you can periodically lightly tap the caliper and hydraulic lines with your fingers. |  |
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3. At the same time we look at the tube extending from the caliper. When the brake line and caliper are full, brake fluid will start pouring out of this tube into the basin. (The caliper and expansion tank are communicating vessels) 4. Close the valve on the caliper with a key. |
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Check that there are no air bubbles left in the hose. | ||
Brakes Is one of the most important elements of a bicycle. They can be mechanical - this is when a cable is used to transfer energy from the handle to the braking mechanism, as well as hydraulic, in this version it is used brake fluid. ()
Basically, each manufacturer recommends their own hydraulic brake fluids, produces a special mineral oil and does not recommend the use of anything else. Hope recommends using DOT 4 or DOT 5.1 automotive brake fluid.
High requirements are imposed on liquids:
1) The liquid should not cause corrosion of metals, as well as destroy oil seals and seals;
2) Liquids must be resistant to low temperatures, must not thicken and freeze;
3) It should also be resistant to high temperatures, should not expand and boil when heated;
4) It must be able to chemically bind the water that gets into it (water in the hydraulic brake system not only causes corrosion, but can also boil when the caliper heats up).
Some of the existing brake fluids:
There are two fundamentally different types of fluids that are poured into the brakes - these are mineral oils (Magura, Shimano) and DOT3 / 4/5 brake fluid (Hayes, Hope, Formula).
Since these liquids have different bases, they should never be mixed or replaced. Brake fluid has a more aggressive effect on metals and seals, therefore, brakes designed for mineral oil must not be filled with brake fluid!
Automotive brake fluids.
1. DOT3- glycol base
boiling point - + 205C
<1500
viscosity stability at + 100C -> 1.5
ultra-high hygroscopicity, service life - 10-12 months
2. DOT4- glycol base
boiling point - + 230C (actually - about 260)
stability viscosity at -40C -<1800
viscosity stability at + 100C -> 1.5
high hygroscopicity, service life - 24 months
3. DOT5- silicone base
boiling point - + 260C (actually - up to 300)
stability viscosity at -40C -<900
viscosity stability at + 100C -> 1.5
practically zero hygroscopicity, high lubricating properties, with a complete absence of anti-corrosion properties; service life - up to 15 years
4. DOT5.1- glycol base
boiling point - + 250C (actually - about 280)
stability viscosity at -40C -<900
viscosity stability at + 100C -> 1.5
highest fluidity, low hygroscopicity, service life - up to 5 years
slurry properties are provided by synthetic additives
Mineral oils for hydraulic brakes:
1. Branded oils Shimano and Magura.
2. Motorex Hydraulic Fluid 75
3. Citroen Hydraulic.
4. Any mineral oil from an auto store with the least amount of any additives.
