Removing air from the brake system (bleeding)

Bleeding the hydraulic brake drive is necessary to remove air, which significantly reduces braking efficiency. Air can get into the hydraulic drive due to depressurization of the system during repairs, replacement of individual components or brake fluid. By partially disconnecting the elements of the hydraulic system and taking appropriate precautions to prevent further loss of fluid, only part of the system can be bled. For maximum safety, bleed the entire brake system. Use only fresh brake fluid (specification SAE J 1703 DOT 3) and clean transparent plastic hose (from the windshield wiper or ventilation of the home aquarium). Work should be done with an assistant. Park the vehicle on a level, level surface.

WARNING

Brake fluid is poisonous - be careful when handling it. In addition, it dissolves paint and plastic, so if liquid gets on the car's paintwork, wash it with plenty of water.



Pic. 14.11. Installing a container with brake fluid 30 cm above the bleeder valve when removing air from the front brake caliper

In the process of bleeding the reservoir must be constantly filled with fresh brake fluid to the mark «MAX». Make sure that the fluid level does not fall below the mark «MIN», otherwise air will enter the system again through the filler hole. When removing air, install a container with brake fluid 30 cm above the bleeder valve - the bleeder valve will be under fluid pressure, and air will not be able to penetrate through the thread of the connector into the brake system (pic. 14.11).




SEQUENCE OF WORK



Pic. 14.12. Disconnecting the connector (1) brake fluid level sensor from plug (2)

1. Disconnect the central connector 1 (pic. 14.12) the sensor of level of a brake liquid from a stopper 2. Turn away a stopper.

2. Bleeding sequence: front left wheel, rear right wheel, front right wheel, rear left wheel.

3. Remove the protective caps and clean the bleeder fittings.



Pic. 14.13. The location of the bleeder fitting with a clean transparent hose put on it


4. Put a clean transparent hose on the bleeder valve, lower the other end into a container partially filled with brake fluid (pic. 14.13).

5. Loosen the bleed screw 1/2-1 turn. The assistant should slowly press the brake pedal, while the liquid in the system, together with air, is forced out through the hose into the container. Then quickly release the pedal to its original position.

6. Wait approximately 3 seconds for the brake master cylinder to fill with brake fluid.

7. Repeat these steps until no more air comes out of the hose. Keep the brake pedal depressed.

8. Screw the bleeder in until it stops and release the brake pedal. Remove the hose from the bleeder plug and add brake fluid to the reservoir.

9. Repeat this operation on all wheels.

10. Add brake fluid to the reservoir up to the mark «MAX», tighten the plug and connect the connector.

11. Carefully check the operation of the brakes during a test drive.



TECHNICAL DICTIONARY

Brake fluid

Brake fluid is a mixture of glycol, polyglycol ether and several other components. It is a yellowish liquid, very toxic and aggressive to car paintwork, but it does not destroy metal and rubber parts. The operating range in which the liquid retains its properties is from -40 to +270°C.


Brake fluid is hygroscopic, i.e. absorbs moisture from the air, so it must be stored in tightly sealed containers. Moisture can penetrate into the brake fluid filled into the hydraulic brake system, for example, from the air through the reservoir or microscopic leaks on the brake pipes and hoses. When water enters the brake fluid, its properties change. Absorption of moisture leads not only to corrosion of the metal parts of the brake system, but also significantly lowers its boiling point. Already with a water content of 2.5% in the brake fluid, its boiling point is approximately 150°C. This is especially dangerous during heavy braking, as the brakes can become very hot in this case, and steam bubbles in the brake fluid can form near the hot brakes. The bubbles are compressed, resulting in increased brake pedal travel, and in some cases, the pedal may even fail. In this case, quickly pumping pressure with the brake pedal will help. This effect is especially dangerous after intensive braking when the car is stopped, because due to the lack of oncoming air flow, the brake mechanisms heat up even more and such a high temperature persists for a long time.

Therefore, for safety reasons, change the brake fluid regularly. If you regularly carry out maintenance of the car in a car repair shop, changing the brake fluid is included in routine maintenance; If you service your vehicle yourself, you should change the brake fluid at least once every two years.

Disposal of old brake fluid

Brake fluid used in hydraulic brakes is poisonous and should be washed off immediately with plenty of water if it comes into contact with the skin. If the liquid gets into an open wound or inside the body, you should immediately consult a doctor.

Brake fluid does not damage metal and rubber parts, but is aggressive to the car's paintwork. If you have drained the brake fluid from the system, it is not recommended to apply it again. Do not use brake fluid from a container that has been open for a long time. Dispose of the old brake fluid in places specially designated for this purpose.