Hydraulic Control Assembly - Antilock Brakes: Service and Repair

Hydraulic unit, brake booster/master cylinder, overview

CAUTION! Before working on the brake system, read CAUTIONS and WARNINGS on go to Service Precautions.

Note: Complete master cylinders and brake boosters can be replaced separately.





1 Brake booster
^ On gasoline engines vacuum is taken from the intake manifold.
^ Diesel engines have a vacuum pump to create the required vacuum.
Functional check:
With engine switched off, depress brake pedal firmly several times (to exhaust the vacuum in the unit).
Depress brake pedal with average foot pressure, hold and start engine. If brake booster is working properly , pedal should give noticeably underfoot (booster assistance becomes effective).
^ If pedal does not give, replace brake booster.
^ Check valve (in vacuum hose) go to Power Brake Assist.
Functional check go to Power Brake Assist.
^ Separating from brake pedal go to Power Brake Assist.
^ Removing and installing go to Power Brake Assist.
2 Cap
3 Brake fluid reservoir
4 Sealing plug
^ Moisten with brake fluid and press into reservoir
5 Retainer pin
^ Insert through brake master cylinder
6 Brake master cylinder
^ Cannot be repaired. If faulty, replace as complete unit
7 Self-locking nut
^ Tighten to 20 Nm (15 ft. lbs.)
^ Always replace
8 Bolt
^ Tighten to 9 Nm (80 inch lbs.)
9 Cap nut
^ Tighten to 9 Nm (80 inch lbs.)
10 ABS/EDL hydraulic unit
11 ABS control module
12 Brake line connection
^ Hydraulic unit to left front brake caliper
13 Brake line connection
^ Hydraulic unit to right rear wheel cylinder/caliper
14 Brake line connection
^ Hydraulic unit to left rear wheel cylinder/caliper
15 Brake line connection
^ Hydraulic unit to right front brake caliper
16 Brake line
^ Brake master cylinder/primary piston circuit to hydraulic unit
17 Brake line
^ Master brake cylinder/secondary piston circuit to hydraulic unit
18 Self-locking nut
^ Tighten to 20 Nm (15 ft. lbs.)
^ Always replace
19 Heat shield Only for vehicles with 110 kW engine
20 Self-locking nut
^ Tighten to 20 Nm(15 ft. lbs.)
^ Always replace
21 Seal
^ Always replace
22 Seal
23 Vacuum hose
^ Insert into brake booster
24 Gasket
^ For brake booster
25 Self-locking nut
^ Tighten to 20 Nm (15 ft. lbs.)
^ Always replace
26 Boot
^ Be sure boot is properly seated.
27 Bracket

Brake lines from brake master cylinder to hydraulic unit, connecting





Fig. 1 Vehicles with ABS
A Secondary piston circuit
B Primary piston circuit
1 Hydraulic unit to right front brake caliper
2 Hydraulic unit to left rear wheel cylinder/brake caliper
3 Hydraulic unit to right rear wheel cylinder/brake caliper
4 Hydraulic unit to left front brake caliper





Fig. 2 Vehicles with ABS/EDL
A Secondary piston circuit
B Primary piston circuit