1. Insert screw plugs into the brake line bleed holes and tighten them so that they do not allow air to pass through, but at the same time they can be easily loosened.
2. Fill the reservoir with brake fluid.
3. Remove one plug and insert the piston into the cylinder to expel the air. You can press on the piston with a large screwdriver.
4. To prevent air from leaking back into the master cylinder, replace the plug and tighten it before releasing the pressure on the piston.
Note: Repeat this procedure until the fluid flowing out of the outlet hole no longer contains air bubbles, then move to the next hole and repeat the entire procedure again. Make sure that the brake fluid reservoir does not run empty, otherwise air may enter the system.
5. Place a new O-ring on the booster, install the master cylinder and mounting nuts and tighten them to the specified torque.
(Material borrowed from an online resource mercedesman)
6. Connect the tubes and electrical connectors. Check the operation of the brake fluid level sensor by pressing the black bulb at the top of the reservoir. The light on the dashboard should light up.
7. On models with manual transmission, bleed the clutch hydraulic system.
8. On models equipped with a differential pressure indicator, press the zero reset pin protruding from the switch.
9. Bleed the brake system.