LAND ROVER DISCOVERY 2002 Workshop Manual

BRAKES
70-6 DESCRIPTION AND OPERATION
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.
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
11Filter12Gaiter
13Piston
14Ratio disc
15Reaction disc
16Central plate
17Master cylinder
18Return spring
19Guide tube
20Output push rod
21Bush
22Air pressures
aPartial vacuum
bAmbient
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.

BRAKES
70-8 DESCRIPTION AND OPERATION
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
11Filter12Gaiter
13Piston
14Ratio 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 connection
4Oil 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.

BRAKES
70-12 DESCRIPTION AND OPERATION
Brakes applied
When the brake pedal is pressed, the output rod in the brake servo assembly pushes the primary piston along the
cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring,
overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial
movement of the pistons, away from the piston stops, closes the primary and secondary centre valves. Further
movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the
brake circuits. The fluid in the chambers behind the pistons is unaffected by movement of the pistons and can flow
unrestricted through the feed holes between the chambers and the reservoir.
Brakes released
When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the
cylinder. The rapid movement of the pistons cause partial vacuums to form in the pressure chambers, which opens
the centre valves and allows fluid to circulate unrestricted between the two hydraulic circuits and the reservoir. When
the pistons reach the brakes off position, the centre valves are held open by the piston stops.

BRAKES
DESCRIPTION AND OPERATION 70-13
ABS modulator
ABS modulator schematic
1Master cylinder/brake servo assembly
2Brake pedal
3ABS modulator
4Shuttle valve switch
5Shuttle valve
6Damper chamber
7Restrictor8Return pump
9Non return valve
10Expansion chamber
11Outlet solenoid valve
12Inlet solenoid valve
13Rear brake
14Front brake

BRAKES
70-14 DESCRIPTION AND OPERATION
The ABS modulator is a 4 channel unit that controls the supply of hydraulic pressure to the brakes in response to
inputs from the SLABS ECU. The modulator is attached by three mounting bushes to a bracket on the LH inner front
wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly.
Three electrical connectors link the ABS modulator to the vehicle wiring.
Passages within the ABS modulator, separated into primary and secondary circuits, connect to the various internal
components that control the supply of hydraulic pressure to the brakes:
lShuttle valves and non return valves control the flow through the internal circuits.
lShuttle valve switches, connected in series to the SLABS ECU, provide a brakes on/off signal.
lA damper chamber and restrictor are included in each circuit to refine system operation.
lInlet and outlet solenoid valves control the flow to the individual brakes.
lAn expansion chamber is connected to each circuit to absorb pressure.
lA return pump is connected to both circuits to provide a pressure source.
The ABS modulator has three operating modes: Normal braking, ABS braking and active braking.
Normal braking mode
When the brake pedal is pressed, pressurised fluid from the master cylinder assembly moves the shuttle valves to
open lines 'A' and close the shuttle valve switches. Pressurised fluid then flows through the open inlet solenoid valves
to operate the brakes. The closed shuttle valve switches supply a brakes on signal to the SLABS ECU. If the SLABS
ECU determines that EBD is necessary, it energises the inlet solenoid valves for the brakes of one axle. The inlet
solenoid valves close to isolate the brakes from any further increase in hydraulic pressure.
ABS braking mode
When in the normal braking mode, if the SLABS ECU determines that ABS braking is necessary, it energises the inlet
and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate
the brake from pressurised fluid; the outlet solenoid valve opens to release pressure from the brake into the expansion
chamber and the return pump circuit. The brake releases and the wheel begins to accelerate. The SLABS ECU then
operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the brake and apply the
maximum braking effort (for the available traction) without locking the wheel.
Active braking mode
When ETC or HDC are enabled, and the SLABS ECU determines that active braking is necessary, it starts the return
pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, shuttle valves and lines 'B', is
pressurised by the return pump and supplied to lines 'A'. The SLABS ECU then operates the inlet and outlet solenoid
valves to control the supply of hydraulic pressure to the individual brakes and slow the wheel(s).

BRAKES
DESCRIPTION AND OPERATION 70-15
Front brakes
1Caliper body
2Bleed screw
3Bleed screw cap
4Piston seal
5Piston
6Piston dust cover
7Brake pad8Caliper carrier
9Brake disc
10Mudshield
11Guide pin bolt
12Guide pin
13Guide pin dust cover
The front brakes each comprise of a hub mounted, twin piston caliper assembly and a ventilated disc. The inboard
side of the disc is protected by a mudshield.
When hydraulic pressure is supplied to the caliper, the pistons extend and force the inner pad against the disc. The
caliper body reacts and slides on the guide pins to bring the outer pad into contact with the disc.