Removing
Note: Please read the warning at the beginning before operation paragraph 2 about the dangers of hydraulic fluid.
1. Disconnect the negative battery terminal.
2. Disconnect the connector from the brake fluid level sensor.
3. Unscrew the master cylinder reservoir cap and remove the hydraulic fluid from the reservoir using a syringe. You can unscrew the brake system air release valve and, by gently pressing the brake pedal, force the fluid into the substituted container through the tube connected to the valve (see paragraph 2).
Warning: Do not aspirate liquid by mouth as it is poisonous; use syringe.
4. Where necessary, disconnect the hydraulic hoses from the side of the reservoir and plug the ends of the hoses to reduce fluid loss.
5. Carefully remove the fluid reservoir from the master cylinder. Remove the reservoir seal and plug the cylinder bores to keep dirt out.
6. Wipe down the area around the tubing fittings on the side of the master cylinder, and lay absorbent rags under the tubing fittings to catch any spillage. Write down the location of the fittings, then unscrew the nuts of the fittings and carefully pull out the tubes. Plug or cover the ends of the tubes and bores in the cylinder to reduce fluid loss and prevent contamination of system parts. Wash off spilled hydraulic fluid immediately with cold water.
7. Loosen and remove the two nuts and washers securing the master cylinder to the vacuum brake booster, then remove the assembly forward from the booster.
Note: Do not tilt the master cylinder until after you have disconnected the pusher from the vacuum booster, otherwise the booster may be damaged.
Remove the O-ring from behind the master cylinder.
Repair
Note: Before dismantling the master cylinder, check the availability of spare parts with a Mercedes-Benz dealer, given that two types of master cylinders are installed on vehicles.
8. Remove all traces of dirt or dust from the cylinder body.
9. Remove the reservoir seals from the master cylinder, noting the correct location of each seal (see fig. 10.9).
Pic. 10.9. Cross section of master cylinder assembly - Teves model (other models are similar]
1 Support seals
2 O-ring
3 Main piston
4 Stop washer
5 Retaining ring
6 Seal
7 Sleeve
8 Washer
9 Seal
10 Seal
11 Spring cup
17 Spring
20 Secondary piston
21 Spring
22 Cylinder body
24 Valve spring
25 Valve seal
27 Secondary piston rod
28 Valve stem
B Fluid inlet
With Compensation hole
D Filling hole and compensatory
10. Carefully clamp the cylinder in a soft-jawed vise, then using a drift, press the piston into the cylinder so that the retaining rod can be removed from the reservoir nozzle.
11. Press the piston back into the cylinder and remove the circlip from the master cylinder, along with its washer and o-ring (where it is installed).
12. Remembering the correct location of each part, remove the main piston assembly and spring from the cylinder.
13. Tap the master cylinder body on a clean wooden surface and remove the secondary piston and spring from the cylinder, noting how the piston is installed.
14. Thoroughly clean all parts using ethyl or isopropyl alcohol or clean brake fluid. Never use petrol or kerosene based solvents as they attack the rubber parts of the hydraulic system parts. Dry parts immediately using compressed air or a clean, lint-free cloth. Use compressed air to blow out the fluid passages.
15. Check the condition of the parts and replace any that are worn or damaged. Especially check the cylinder bores and pistons; they need to be replaced (note that the assembly changes in assembly) if they are scratched, worn or corroded. Similarly, check the condition of the fingers and their bushings; both fingers must be intact, and (when thoroughly cleaned) tight enough to be installed in the bushings. If there is any doubt about the condition of any part, replace it.
16. If the unit can be used further, purchase an appropriate repair kit from a Mercedes-Benz dealer. It is absolutely necessary to replace all rubber seals; they cannot be reused.
17. Dip piston and all seals in clean hydraulic fluid prior to installation. Lubricate the surface of the cylinder bore with clean fluid.
18. Install the spring on the secondary piston assembly and insert the piston into the master cylinder body, making sure it is seated correctly. Install the piston by rotating it from side to side, checking that the piston seals are not pinched at the cylinder inlet.
19. Install the secondary piston so that the groove on the piston is correctly positioned in relation to the main cylinder body, then press the piston into place and secure it with a retaining ring.
20. Install the new master cylinder assembly as described in step 18.
21. Where provided, install a new o-ring on the end of the master cylinder, then install the washer and circlip. Make sure the circlip is correctly seated in the master cylinder groove.
22. Install new master cylinder reservoir seals to the master cylinder nozzles.
Installation
23. Remove all traces of dirt and dust from the master cylinder and brake booster. Install a new O-ring on the back of the master cylinder body.
24. Install the master cylinder to the brake booster, making sure the piston enters the center of the master cylinder bore. Install the cylinder nuts and tighten them to the required torque.
25. Wipe the union of brake pipes, then establish them on the main cylinder and tighten with the demanded effort.
26. Mount the reserve reservoir on the cylinder and, where necessary, connect the hydraulic hoses.
27. Fill the master cylinder reservoir with new fluid and bleed the entire hydraulic system as described in paragraph 2. Thoroughly check the operation of the brake system before driving on the road.