Discovery 2. Manual — part 207

BRAKES

DESCRIPTION AND OPERATION

70-3

1 ABS sensor
2 ABS modulator
3 Return pump relay
4 SLABS ECU
5 Brake lamp relay
6 Centre high mounted stop lamp
7 LH brake lamp
8 RH brake lamp
9 Instrument pack

10 Body control unit
11 Reverse lamp switch (manual gearbox)
12 Diagnostic socket
13 HDC switch
14 Transmission high/low switch
15 Centre differential lock switch
16 Engine control module
17 Battery power supply
18 Ignition power supply

BRAKES

70-4

DESCRIPTION AND OPERATION

Brake system hydraulic component
layout

RH drive shown, LH drive similar

1 Rear brake
2 ABS modulator
3 Front brake
4 Inlet manifold plenum (V8 models)
5 Non return valve

6 Master cylinder assembly
7 Brake fluid reservoir
8 Brake servo assembly
9 Vacuum pump (diesel models)

BRAKES

DESCRIPTION AND OPERATION

70-5

Description

General
The brakes consist of front and rear disc brakes operated by a diagonally split, dual circuit hydraulic system with
vacuum servo power assistance. The system incorporates the following control functions as standard on all models:

l

Anti-lock Brakes (ABS), to prevent road wheels locking during brake application.

l

Electronic Brake Distribution (EBD), to control distribution of hydraulic pressure between front and rear axles.
Replaces mechanical pressure limiting valve of previous systems.

l

Electronic Traction Control (ETC), to maintain even torque distribution to the road wheels.

l

Hill Descent Control (HDC), to provide controlled descent ability in off road conditions.

Hydraulic system schematic

1 Master cylinder/brake servo assembly
2 Brake pedal
3 ABS modulator
4 Rear brake

5 Front brake
6 Hydraulic circuit

a Primary

b Secondary

For normal brake operation, brake pedal movement is assisted by the brake servo assembly and transmitted to the
master cylinder assembly. The master cylinder assembly converts brake pedal movement to hydraulic pressure.
Primary and secondary brake pipe circuits supply the hydraulic pressure to the brakes via the ABS modulator: the
primary circuit supplies the front left and rear right brakes; the secondary circuit supplies the front right and rear left
brakes. Vacuum for the brake servo assembly is obtained from the engine inlet manifold (V8 models) or a vacuum
pump (diesel models), through a vacuum line and non return valve. To reduce operating noise, sleeving is installed
on some of the brake pipes in the engine compartment and the pipes are located in sprung pipe clips.

For all control functions, the ABS modulator regulates the hydraulic pressure to the brakes to control the speed of all
four wheels, either individually or in axle pairs. Operation of the ABS modulator is controlled by the Self Levelling and
Anti-lock Braking Systems (SLABS) ECU. The SLABS ECU also operates warning indications in the instrument pack
to provide the driver with status information on each function.

BRAKES

70-6

DESCRIPTION AND OPERATION

Brake servo assembly
The brake servo assembly provides power assistance to reduce the pedal load when braking. If the brake servo
assembly fails, the hydraulic system still functions but will require greater brake pedal effort due to the lack of vacuum
assistance.

Two integral tie bolts attach the brake servo assembly to the pedal and bracket assembly on the engine bulkhead.
The master cylinder assembly is attached to the forward ends of the tie bolts.

The brake servo assembly consists of a circular housing which contains two diaphragms, a central plate, a control
valve assembly, input and output push rods and a filter. The input push rod is connected to the brake pedal. The output
push rod locates in the primary piston of the master cylinder. A protective gaiter is installed on the control valve
assembly where it extends from the rear of the housing. A non return valve, installed in a port in the front face of the
housing, is connected to a vacuum line from the engine.

The control valve assembly consists of a valve body containing a valve, a piston, a valve spring and an input rod
spring. The valve controls a vacuum port in the valve body. The piston controls an air inlet port between the valve and
the piston. A reaction disc and a ratio disc separate the piston from the output push rod. A guide tube on the front of
the valve body is attached to the front diaphragm and supported in a bush in the central plate. A return spring locates
in the open end of the guide tube.

The two diaphragms and the central plate separate the interior of the housing into four sealed chambers. The
chambers at the front of the diaphragms are connected together through fixed passages in the valve assembly. The
chambers at the rear of the diaphragms are connected together through the interior of minor diaphragms on the tie
bolts.

Brakes off
With the brake pedal released, the piston in the control valve assembly positions the valve so that the vacuum port is
open and the two pairs of chambers are connected together. When the engine is running air is evacuated through the
vacuum line and non return valve, creating a partial vacuum in all four chambers. When the engine stops, the non
return valve closes to maintain the partial vacuum and, on V8 models, prevent fuel vapour entering the brake servo.