Warning! The brake fluid used in the hydraulic clutch system is poisonous, so if it comes into contact with the skin, immediately wash the area with plenty of water. If the liquid gets into the eyes or inside the body, you should immediately consult a doctor. Some fluids tend to be highly flammable and can ignite even from contact with hot vehicle components. In addition, the fluid used in the hydraulic clutch system dissolves paint and plastic, so if fluid comes into contact with the car's paintwork, wash it with plenty of water. Also, this liquid is hygroscopic, i.e. it absorbs moisture from the air and, therefore, it is not recommended to use liquid stored for a long time in an open container.
Removing air from the rear brake caliper
Bleeding the hydraulic brake system is necessary to remove air, which significantly reduces braking efficiency. Air can get into the hydraulic system due to depressurization of the system during repairs, replacement of individual components or brake fluid. The presence of air in the drive is indicated by an increased stroke of the brake pedal and its softness. Before bleeding, check the tightness of all brake drive units and their connections.
Never reuse brake fluid.
When performing the operation of pumping the brake system, do not let the brake fluid level fall below the middle of the tank.
If brake fluid has left the system due to a system leak, find the cause and repair the leak before proceeding.
If the hydraulic system has been partially disconnected and appropriate precautions have been taken to prevent further loss of fluid, only part of the system can be bled.
Bleeding can be done with an assistant or on your own with a brake bleeding kit. Always follow the instructions. It is recommended to use the kit wherever possible, as it greatly simplifies the bleeding procedure. If there is no bleed kit, stock up on a clean container and a clear plastic tube of the appropriate length, which should fit snugly over the bleed fitting. This will require the help of another person.
Pumping sequence
If only a partial connection of the hydraulic brake system has been made and appropriate precautions have been taken, then it is sufficient to bleed only this part of the system.
Bleed the entire brake drive system in the following sequence:
- right rear wheel;
- left rear wheel;
- right front wheel;
- left front wheel.
Attention! On models with speed control (ASR), If the hydraulic system, brake master cylinder, hydraulic block, hydraulic pump and accumulator were repaired, it is also necessary to bleed the hydraulic reservoir.
Pumping - the main method (two people)
Remove the dust cap from the wheel cylinder bleeder, clean it and put a clean transparent hose on it, lower the other end into a container partially filled with brake fluid. The container must be at least 300 mm above the bleeder port. This prevents air from entering the cylinder through the bleed screw threads. Press the brake pedal sharply 3-5 times with an interval of 2-3 seconds, unscrew the bleed valve half a turn while pressing the pedal. Continuing to press the pedal, force the fluid in the system together with air through the hose into a container. After the pedal has reached its full forward position and fluid has stopped flowing through the hose, screw the bleeder in until it stops. Repeat these operations until the air flow from the hose stops.
While holding the pedal down, screw in the bleed screw until it stops and remove the hose.
Wipe the bleeder valve and install the protective cap.
Repeat these operations for other wheels.
Attention! When removing air, constantly monitor the presence of brake fluid in the tank, not allowing its bottom to be exposed, as this will allow air to enter the system again.
If there is no air in the brake system, the pedal should travel about half of its travel. To eliminate the influence of the vacuum brake booster on bleeding the brakes, perform air removal with the engine off.
Bleeding - using a one-way valve kit
The kit consists of a tube with a one-way valve installed on it, which prevents the return of the removed brake fluid with air bubbles into the brake system. On some kits, a transparent container is installed in which air bubbles in the brake fluid are clearly visible (see fig. Removing air from the rear brake caliper).
Remove the dust cap from the wheel cylinder bleeder, clean it and put the hose from the kit on it. Unscrew the bleed screw half a turn. Gently press the brake pedal to the stop and slowly release it to its original position. Repeat this operation until the brake fluid comes out of the bleeder valve without air bubbles.
Tighten the bleeder fitting until it stops, remove the tube from it and put on the dust cap.
Attention! When removing air, constantly monitor the presence of brake fluid in the tank, not allowing its bottom to be exposed, as this will allow air to enter the system again.
Bleeding - Using a Pressure Bleeding Kit
The use of a bleed kit requires compressed air to be supplied to it, for which an inflated spare tire can be used. Reduce the tire pressure to the value specified in the kit's instruction manual.
Attention! Mercedes-Benz recommends the use of compressed air at a maximum pressure of 2 bar.
Connect a container filled with brake fluid to the master cylinder reservoir. The bleed process starts after unscrewing the bleed screw half a turn. After the outflow of brake fluid with air bubbles stops and only clean brake fluid flows out, screw the bleeder in until it stops.
When bleeding this method, a large amount of hydraulic fluid is needed to prevent air from entering the brake master cylinder during the bleeding process, which often occurs if the fluid level in the reservoir is not maintained.
Bleeding is especially effective when the system is difficult to bleed or when the system is completely bled during a normal brake fluid change.
All Methods
After the pumping process is complete, press the brake pedal hard, while it should go through about half of its travel. Wash off all traces of spilled brake fluid and tighten the bleeder fittings to the required torque, then install the dust caps on them.
Check the brake fluid level in the reservoir of the brake system and, if necessary, add brake fluid.
Removed brake fluid should no longer be used. Clean brake fluid must be stored in a sealed container, as it is highly hygroscopic, which lowers the boiling point and degrades brake fluid performance.