Removing and Installing Brake Booster
WM 477019 Removing and installing brake booster
Information
The brake components (brake booster, brake master cylinder, hydraulic unit) are installed under the luggage compartment cover -2-.
Luggage compartment cover
To remove and install the brake booster -C-, the hydraulic unit -D- must be removed.
The brake master cylinder -B- must not be separated from the brake booster -C- if both parts -B and C- are to be re-used.
Brake components
Preliminary work
Preliminary work
1. Remove cover -A-.
Cover over brake push rod
2. Remove cover over the brake booster (front luggage compartment cover).
There are two types of luggage compartment covers -1- (illustrations below).
Removing luggage compartment cover
Removing luggage compartment cover
Removing brake booster
1. Suck brake fluid out of the reservoir or pump out the brake fluid using the brake pedal via a front and rear brake caliper. Use a collecting bottle.
2. Pull electric line -Arrow- off the reservoir -A-. Undo the brake lines -Arrows- on the brake master cylinder -B-. Close the brake lines and connections with suitable plugs.
Brake lines on brake master cylinder
3. Remove the hydraulic unit -D- together with the support -F- and the four brake lines, which are fitted on the intermediate piece -E-.
A - Brake fluid reservoir
B - Brake master cylinder
C - Brake booster
D - Hydraulic unit
E - Intermediate piece
F - Support for hydraulic unit
Brake components
Information
The brake master cylinder must not be separated from the brake booster if both parts are to be re-used.
4. Undo the fastening nuts -1- on the brake master cylinder. Pull out brake master cylinder.
1 - Brake master cylinder mount
2 - Brake booster mount
Brake master cylinder mount
5. Pull the bellows back from the body and remove the lock nut -Arrow-. Separate the two push rods from each other.
Push rod mount
6. Disengage the vacuum line at the brake booster. Unscrew the two fastening screws (Torx) on the brake booster and remove the brake booster.
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.
^ 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 calipers.
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).
Brake booster push rod
1. Insert brake booster with bellows and tightened clamp -Arrow-.
Tighten fastening screws (Torx/M8) to
--> Tightening torque: 23 Nm (17 ftlb.).
Bellows
2. Connect the push rods. Use a new lock nut -Arrow- to do this.
Push rod mount
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 -1-.
Brake pedal position
4. Fit the bellows and secure it to the body with a clamp -Arrow-.
Bellows
5. Fit the vacuum hose on the brake booster.
6. Fit brake master cylinder and reservoir.
Tighten fastening nuts (M14) to --> Tightening torque: 49 Nm (36 ftlb.).
7. Install hydraulic unit -D-. Observe correct tightening torques.
Hydraulic unit
8. Bleed the brake system and adjust the brake-fluid level.
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 corresponding brake calipers.
^ 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.
Subsequent work
1. Install cover over the brake booster (front luggage compartment cover).
There are two types of luggage compartment covers (illustrations below).
Installing luggage compartment cover
Installing luggage compartment cover
2. Install cover -A-.
Cover over brake push rod