Installing brake booster

Information 

• Absolute cleanliness is necessary when installing the brake booster as well as the adjacent brake master cylinder. Dirt can cause faults in the system. 
• The reservoir must engage correctly in the brake master cylinder. 
• Observe correct tightening torques →  TECHNICAL VALUES  ". 
• First bleed the brake system in the usual way (primary circuit). Then, use the PIWIS Tester to bleed the brakes as well (secondary circuit). 
• Check electric and hydraulic lines for incorrect assignment using the PIWIS Tester. This test checks that the brake lines/solenoid valves are assigned to the corresponding brake callipers. 

When replacing the brake booster, observe the correct allocation . Different transmission ratio (boost ratio) and tandem brake booster for four-wheel drive vehicles.

The basic push rod setting, installation dimension -X-  , is already set on new brake boosters. When replacing the push rod, always reset the previous dimension -X-  (approx. 140 mm) if necessary. Fine adjustment is carried out following installation (position of brake pedal with respect to accelerator pedal).

Fig 1: Identifying Brake Booster Push Rod

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1. Insert brake booster with bellows and tightened clamp -Arrow-  .

   Tighten fastening screws (Torx/M8) to → Tightening torque: 23 Nm (17 ftlb.). 

   Fig 2: Locating Bellows

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2. Connect the push rods. Use a new lock nut -Arrow-  to do this.
   Fig 3: Locating Push Rod Mount

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   Information 

   Adjusting the brake push-rod length changes the position of the brake pedal.

   If the brake pedal is not at the correct position, the brake push rod must be adjusted slightly.

   Check the brake light switch setting.

3. Check the position of the brake pedal and adjust it if necessary (Dimension X = 48 +2 mm). Measure between the brake pedal plate and the accelerator pedal.

   Fine adjustment for brake push rod (Dimension X = 48 +2 mm): Undo the lock nut -1-  and turn the push rod as required at the hexagon -2-  , then set the dimension -X-  . Tighten the lock nut.

   Fig 4: Identifying Brake Pedal Position

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4. Fit the bellows and secure it to the body with a clamp -Arrow-  .
   Fig 5: Locating Bellows

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5. Fit the vacuum hose on the brake booster.
6. Fit the brake master cylinder and reservoir.

   Tighten fastening nuts (M14) to → Tightening torque: 49 Nm (36 ftlb.). 

7. Install hydraulic unit -D-  , → 453019 REMOVING AND INSTALLING HYDRAULIC UNIT . Fit all brake lines. Observe correct tightening torques → WM 4700TW TIGHTENING TORQUES FOR BRAKE HYDRAULICS (ALL MODELS)   .
   Fig 6: Identifying Hydraulic Unit

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8. Bleed the brake system and adjust the brake-fluid level → 470107 BLEEDING THE BRAKE SYSTEM .
9. Check the function of the brake light switch. The pedal must move a perceptible distance of approx. 5 to 10 mm before the brake light comes on. At the same time, make sure that the brake light comes on before the pedal meets perceptible resistance or before brake pressure builds up. If necessary, pull the cam follower as far as it will go out of the brake light switch with the brake pedal pressed. This sets the push rod to the exact length when the pedal is pressed.
10. When replacing the hydraulic unit, code the control unit (manual transmission/Tiptronic).
11. Carry out the following tests:
    • Check the system for leaks.
    • Read out and erase the fault memory of the ABS/PSM control unit using the PIWIS Tester. Carry out a static test as well.
    • Check electric and hydraulic lines for incorrect assignment using the PIWIS Tester. This test checks that the brake lines/solenoid valves are assigned to the corrresponding brake callipers.
    • Then start the engine. The ABS/PSM-related lights in the instrument cluster must go out when the engine is running.
    • Carry out a function check on the brakes. Then take a short test drive and perform at least one ABS control operation, taking the road conditions into consideration. Read out the ABS/PSM fault memory again. There must not be any stored faults.