Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Master Cylinder

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CAUTION
Brake pads may contain asbestos, which has been determined to be a cancer causing agent. Wear an approved filter mask or respirator whenever working around brakes, to avoid inhaling dust from any brake surface. Never clean brake components with compressed air! instead, use a commercially available brake cleaning fluid.

The dual master cylinder has a safety feature which the single unit lacksif a leak develops in one brake circuit (rear wheels, for example), the other circuit will still operate.

Failure of one system is immediately obviousthe pedal travel increases appreciably and a warning light is activated. When the fluid falls below a certain level, a switch activates the circuit.


CAUTION
This design was not intended to allow driving the car for any distance with, in effect, a two-wheel brake system. If one brake circuit fails, braking action is correspondingly lower. Front circuit failure is the more serious, however, since the front brakes contribute up to 75% of the braking force required to stop the car.

REMOVAL & INSTALLATION



See Figures 1 through 6

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Fig. Fig. 1: To remove the brake master cylinder, use a turkey baster or similar implement to remove the fluid ...



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Fig. Fig. 2: ... and place the old brake fluid into a suitable container



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Fig. Fig. 3: Disengage any electrical connections from the master cylinder



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Fig. Fig. 4: Using a rag to catch any spilled fluid and a flare nut wrench, disengage the fluid lines



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Fig. Fig. 5: Loosen the master cylinder-to-brake booster nuts ...



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Fig. Fig. 6: ... then remove the master cylinder from the engine compartment

  1. To remove the master cylinder, use a tool such as a turkey baster to remove the brake fluid from the reservoir.
  2.  
  3. Disconnect the switch connectors using a small screwdriver. Disconnect the brake lines at the master cylinder. Plug the ends with bleed screw caps or the equivalent.
  4.  
  5. Unbolt the master cylinder from the power brake unit and remove. Be careful you do not lose the O-ring in the flange groove of the master cylinder.
  6.  
  7. Installation is the reverse of removal. Be sure to replace the O-ring between the master cylinder and the power brake unit, since this must be absolutely tight. Torque the nuts to 12-15 ft. lbs. Be sure that both chambers are completely filled with brake fluid and bleed the brakes.
  8.  


WARNING
Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded.

OVERHAUL



See Figures 7 and 8

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Fig. Fig. 7: Cross-sectional view of a common step-type master cylinder1976-84 models



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Fig. Fig. 8: Cross-sectional view of the master cylinder used on 190D and 190E models

  1. To disassemble pull the reservoir out of the top of the cylinder.
  2.  
  3. Remove the screw cap, strainer, and splash shield.
  4.  
  5. Unscrew the cover caps and take out the inserts and O-rings.
  6.  
  7. Push the piston inward slightly and remove the stop screws.
  8.  
  9. Remove the piston stop-ring in the same manner, then pull out the piston and other components.
  10.  
  11. The spring must be unscrewed from the piston.
  12.  
  13. Clean all parts in clean brake fluid.
  14.  
  15. Check the housing bore for score marks and rust. Do not hone the cylinder bore. If slight rust marks do not come out with crocus cloth, replace the housing.
  16.  
  17. Assembly is the reverse of disassembly. Before installing the pistons, coat the sleeves of both pistons with brake fluid.
  18.  

Do not force the pistons into the housings. A special tool is available to install the pistons, but if it is not available, install the pistons very carefully with a slight twisting motion. The special assembly tools can be fabricated in the shop from light metal alloy, according to the dimensions given.

BRAKE BLEEDING



See Figures 9 and 10

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Fig. Fig. 9: 1. Cap 2. Bleeder screw To bleed the brake system, first remove the bleeder screw cap (if equipped)



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Fig. Fig. 10: Open the bleeder screw and bleed the old fluid through a hose into a container partially filled with clean fluid


WARNING
Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded.

Always bleed the brakes after performing any service, or if the pedal seems spongy (soft). The location of the bleed screws can be seen by consulting the illustrations throughout this section. Prior to bleeding each wheel, connect a hose to the bleed screw and insert the hose into a jar of clean brake fluid.

On dual master cylinders, bleed only the circuit that has been opened. If both circuits have been opened, first bleed the circuit connected to the pushrod bore starting with the wheel farthest from the master cylinder, then bleed the other circuit.

  1. First have an assistant pump the brakes and hold the pedal.
  2.  
  3. Then, starting at the point farthest from the master cylinder, slightly open the bleed screw.
  4.  
  5. When the pedal hits the floor, close the bleed screw before allowing the pedal to return (to prevent air from being sucked into the system).
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  7. Continue this procedure until no more air bubbles exit from the bleed screw hole, then go to the next wheel. Fluid, which has been bled from the system, is filled with microscopic air bubbles after the bleeding process is completed, therefore it should be discarded.
  8.  

 
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