1999 LAND ROVER DISCOVERY Workshop Manual

BRAKES
70-4 DESCRIPTION AND OPERATION
Brake system hydraulic component
layout
RH drive shown, LH drive similar
1Rear brake
2ABS modulator
3Front brake
4Inlet manifold plenum (V8 models)
5Non return valve6Master cylinder assembly
7Brake fluid reservoir
8Brake servo assembly
9Vacuum 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:
lAnti-lock Brakes (ABS), to prevent road wheels locking during brake application.
lElectronic Brake Distribution (EBD), to control distribution of hydraulic pressure between front and rear axles.
Replaces mechanical pressure limiting valve of previous systems.
lElectronic Traction Control (ETC), to maintain even torque distribution to the road wheels.
lHill Descent Control (HDC), to provide controlled descent ability in off road conditions.
Hydraulic system schematic
1Master cylinder/brake servo assembly
2Brake pedal
3ABS modulator
4Rear brake5Front brake
6Hydraulic circuit
aPrimary
bSecondary
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.

BRAKES
DESCRIPTION AND OPERATION 70-7
Section through brake servo assembly - brakes off
1Non return valve
2Housing
3Diaphragms
4Minor diaphragm
5Tie bolt
6Valve body
7Valve
8Valve spring
9Input rod spring
10Input push rod
11Filter
12Gaiter13Piston
14Ratio disc
15Reaction disc
16Central plate
17Master cylinder
18Return spring
19Guide tube
20Output push rod
21Bush
22Air pressures
aPartial vacuum
bAmbient

BRAKES
70-8 DESCRIPTION AND OPERATION
Brakes on
When the brake pedal is pressed, the input push rod and the piston initially move forward in the valve body. The valve
body and output rod then move with the input rod, against resistance from the return spring, to transmit the brake
pedal force to the master cylinder assembly.
During the initial movement of the piston, the valve, assisted by the valve spring, moves with the piston and closes
the vacuum port to isolate the chambers at the rear of the diaphragms from the vacuum source. Further movement
of the input push rod causes the piston to move away from the valve and open the air inlet port. This allows a restricted
flow of filtered ambient air through the air inlet port, which creates a servo pressure in the chambers at the rear of the
diaphragms. Force from the resultant pressure differential across the diaphragms is transmitted through the valve
body to the output push rod, augmenting the pressure being applied by the brake pedal. The force produced by the
diaphragms, in proportion to the input force, i.e. the boost ratio, is 5.6 : 1. The boost ratio remains constant, as the
input force from the brake pedal increases, until the limit of assistance is reached when servo pressure is equal to
ambient pressure.
Brakes held on
When the brake pedal effort is constant, opposing pressures cause the reaction disc to extrude onto the ratio disc,
which moves the piston against the valve to close the air inlet port. This prevents any further increase in servo
pressure and maintains a constant output force to the master cylinder assembly.
Brakes released
When the brake pedal is released, the input rod spring moves the input rod and piston rearwards within the valve body
to close the air inlet port and open the vacuum port. The air from the chambers at the rear of the diaphragms is then
evacuated, through the vacuum port and the chambers at the front of the diaphragms, to restore a partial vacuum in
all four chambers. Simultaneously, the return spring moves the valve body, diaphragms, output rod and input rod
rearwards to return them to their brakes off position.

BRAKES
DESCRIPTION AND OPERATION 70-9
Section through brake servo assembly - brakes on
1Non return valve
2Housing
3Diaphragms
4Minor diaphragm
5Tie bolt
6Valve body
7Valve
8Valve spring
9Input rod spring
10Input push rod
11Filter
12Gaiter
13Piston14Ratio disc
15Reaction disc
16Central plate
17Master cylinder
18Return spring
19Guide tube
20Output push rod
21Bush
22Air pressures
aPartial vacuum
bAmbient
cServo

BRAKES
70-10 DESCRIPTION AND OPERATION
Vacuum pump (diesel models only)
1Alternator
2Vacuum pump
3Brake servo assembly vacuum connection4Oil feed pipe
5Oil drain pipe
As the diesel engine air inlet system does not produce sufficient depression to operate the brake servo assembly, an
engine driven vacuum pump is installed.
The vacuum pump is integrated with the engine alternator and driven by the auxiliary drive belt. The pump is a rotary
vane type, lubricated and cooled by engine oil supplied through a pipe connected to the engine block and returned
through a pipe connected to the engine oil sump. Air extracted from the brake servo assembly is vented into the oil
sump with returning lubricating oil.

BRAKES
DESCRIPTION AND OPERATION 70-11
Master cylinder assembly
Section through master cylinder assembly
1Reservoir filler cap
2Switch float
3Brake fluid reservoir
4Primary feed hole
5Valve pin
6Primary centre valve
7Primary piston
8Primary spring9Pressure chamber
10Cylinder
11Valve pin
12Secondary centre valve
13Secondary spring
14Pressure chamber
15Secondary piston
16Secondary feed hole
The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed.
The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons
in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the
secondary circuit. A brake fluid reservoir is installed on top of the cylinder. The reservoir is internally divided to provide
an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and
secondary brake circuits. Should a failure occur in one circuit, the remaining circuit will still operate effectively,
although brake pedal travel and vehicle braking distances will increase. If the fluid level in the reservoir is too low, a
float operated switch in the reservoir filler cap connects an earth to the instrument pack, which illuminates the brake
warning lamp.