Hydraulic Control Assembly - Antilock Brakes: Service and Repair
Hydraulic unit, brake servo/brake master cylinder, assembly overview from model year 2000: Vehicles from VIN 3B Y 000 001
1 - Tandem brake servo
^ On gasoline engines the vacuum required is taken from the intake manifold.
^ From model year 2001, vehicles with a gasoline engine and automatic transmission are equipped with a brake vacuum pump Page 47-57
^ Function check
- With engine switched off, depress brake pedal firmly several times (to exhaust the vacuum in the module).
- Depress brake pedal with average foot pressure, hold and start engine. If the servo module is working properly, the pedal will drop slightly (servo assistance becomes effective).
^ If faulty replace.
2 - Brake line connection
^ To left rear caliper
^ Marked on hydraulic unit with -HL-
3 - Brake line connection
^ To right-rear caliper
^ Marked on hydraulic unit with -HR-
4 - Brake line
^ Master brake cylinder/secondary piston circuit to hydraulic unit
5- Brake line
^ Brake master cylinder/primary piston circuit to hydraulic unit
6 - Brake line
^ To left-front brake caliper
^ Marked on hydraulic unit with -VL-
7 - Hex nut, 12 Nm
8 - Threaded stud
9 - Rubber damper
10 - ABS hydraulic unit
11 - Brake line
^ To right front brake caliper
^ Marked on hydraulic unit with -VR-
12 - Retainer
13 - Hex bolt, 9 Nm
14 - Retainer
^ Allocation Parts catalog
15 - Rubber damper
16 - Hex bolt
17 - Tandem brake master cylinder
^ Cannot be repaired. If faulty, replace.
18 - T45 torx socket head bolt, 25 Nm
19 - Selflocking nut, 49 Nm
20 - Sealing plug
^ Coat with brake fluid and press into brake fluid reservoir
21 - Brake fluid reservoir
22 - Oil seal
23 - Cap
24 - Vacuum hose
^ Insert into brake servo module
25 - Oil seal
Connecting tandem brake master cylinder brake lines to the hydraulic unit for ABS/EDL front and All Wheel Drive (AWD) and ABS/EDL/ASR Front Wheel Drive (FWD)
A- Secondary piston circuit
B - Primary piston circuit
1 - Hydraulic unit to left-rear parking brake caliper
2 - Hydraulic unit to right-rear parking brake caliper
3 - Hydraulic unit to right-front brake caliper
4 - Hydraulic unit to left-front brake caliper
Control module and hydraulic unit removing and installing
Special tools and workshop equipment required
VAG 1331 torque wrench (or equivalent 5 to 50 Nm)
VAG 1869/2 brake pedal loading device
The hydraulic unit and control module are not available as individual items. If found to be faulty, the hydraulic unit and control module should always be removed together.
Location:
The control module is bolted to the hydraulic unit and is located on the left side, in the engine compartment.
WARNING: Do not bend the brake lines in the area of the hydraulic unit!
Removing:
^ Vehicles equipped with coded radio, note or request radio code if necessary.
^ Disconnect battery.
^ Release control module connector (arrow -1-) and pull off (arrow -2-).
^ Connect bleeder bottle hose to left-front brake caliper bleed screw and open bleed screw.
^ Install brake pedal loading device from VAS 5234
^ Depress brake pedal with brake pedal depressor.
^ Close left-front bleeder screw.
^ Place lint-free cloths under the control module and hydraulic unit.
^ Disconnect brake lines on the hydraulic unit.
^ Seal brake lines and threaded holes with plugs from repair kit Part No. 1 HO 698 311 A.
^ Remove hex nuts on hydraulic unit bracket -arrows-.
^ Remove hydraulic unit.
Installing:
Notes:
^ Remove sealing plugs on new hydraulic unit when the corresponding brake line is going to be connected.
^ If the sealing plugs are removed too early, brake fluid can escape, and can no longer be guaranteed that the unit is filled or bled.
Installation is the reverse of Removal
Bleeding brake system
^ Enter radio code.
^ Code control module
Code control module using VAS 5051 via "Guided fault finding" function.
Tightening torques:
Brake lines to ABS module:
Thread M 10 x 114 Nm
Thread M 12 x 114 Nm
Hydraulic unit hex nut to retainer