brake LAND ROVER DISCOVERY 1995 Workshop Manual

BRAKES
7
DESCRIPTION AND OPERATION BRAKE SERVO UNIT
Description
The power assistance for the brake system is
provided by a twin diaphragm suspended vacuum
servo.
A 'Suspended Vacuum' servo is one where, with the
engine running vacuum is present both sides of the
diaphragm or diaphragms when the brakes are fully
released or partially applied. The principal of having
vacuum both sides of the diaphragm has two main
advantages. First, it ensures instantaneous servo
response to the brake being applied or released and
second, it provides very accurate control of the
position of the diaphragm and therefore, the degree of
partial brake application.At full pedal depression, vacuum is present in front of
the diaphragm with atmospheric pressure on the rear.
Operation
Brakes released
With the brakes released depression is present both
sides of the diaphragms, in chambers 1,2,3 and 4.
The chambers comunicate by ports A, B, D via the
valve C which is shown fully released in RR3640M.

70BRAKES
8
DESCRIPTION AND OPERATION Brakes applied
When the brake pedal is depressed the pedal pushrod
operates valve C situated in the plastic diaphragm hub
9 and 10. Inital depression of the the brake pedal,
push rod 6 and plunger 7 compress a rubber pad 8.
This slight movement causes valve C to close port D
sealing off the vacuum to chambers 2/4 and allows
atmospheric pressure to enter the chambers through
the air filter E.
With depression in chambers 1/3 and air pressure in
chambers 2/4 the servo hub will apply the brakes via
the master cylinder push rod 13.Atmospheric pressure over the large area of the
diaphragms multiples the force applied to the master
cylinder piston, to provide the power assistance. If the
brake pedal is only partially depressed, the servo hub,
diaphragms and master cylinder push rod will stop
moving, when valve C comes to rest on the plunger 7.
At this point the valve will balance the pressure in all
the chambers, to the applied effort at the pedal and
provide proportional braking.
It is only when the brakes are fully applied that the
valve does not balance the pressures in all the
chambers, but ensures that maximum available
depression is in chambers 1/3 and full atmospheric
pressure enters chambers 2/4.
When the brake pedal is released, the pressure in all
chambers is equalised and the servo is returned to the
rest position by spring 12.

BRAKES
9
DESCRIPTION AND OPERATION Petrol engine vacuum system
The vacuum necessary to operate the brake servo on
petrol engine vehicles is provided by the engine inlet
manifold. During normal operation of the brakes only a
small volume of air is drawn from the servo and into
the inlet manifold or plenum chamber. However any
excessive air leaks due to damage in the brake servo
system or manifold hose connection, will effect the
mixture strength and ECM fuelling calculations on
engines equipped with an MFI system.
Diesel engine vacuum system
As the diesel engine inlet system does not produce
depression sufficient to operate the brake servo, it is
necessary to install an engine driven vacuum pump.
During normal operation of the brakes only a small
volume of air is drawn from the servo by the vacuum
pump. However as the pump exhausts air into the
engine crankcase atmosphere, an air leak due to
damage in the brake servo system or pump hose
connection, may cause excessive crankcase
pressure.

BRAKES
1
FAULT DIAGNOSIS ABS FAULT DIAGNOSIS
If a fault has occurred, or has been identified by ECU
self diagnostic function and ABS warning light is
illuminated. The system and components must be
checked to locate and rectify fault, using Testbook
diagnostics.
NOTE: If warning lamp has indicated a
fault in system, and no fault code has been
stored in memory, cause of fault is:
a) Failure in electrical supply
b) Bad ECU ground
c) Faulty warning light relay
d) ECU not connected
Before commencing fault diagnosis procedure
following items must be checked:
1.Inspect all exposed cables for damage or
abrasion.
2.Check ground on ABS system.
3.Battery - state of charge.
4.Check hub end-float.
5.All ABS fuses and electrical connections.
Fault rectification
1.Complete harness should be replaced if faults
are found in wiring harness.
2.DO NOT use unspecified cables or connectors,
as this could jeopardise safe function of ABS.
3.DO NOT attempt to open sealed 35 way
connector to ECU.FAULT DIAGNOSIS PROCEDURE
NOTE: If ABS warning light illuminates due
to large sensor air gap, fault will be
retained by the ECU memory. Where wheel
sensors have been pushed fully home prior to
test, The ECU will indicate a fault that has been
rectified.
NOTE: After any steering adjustment,
bearing replacement/adjustment, brake
disc replacement: Check hub end-float and
sensor clearance.
RELAYS AND FUSES ABS
The location and identification of ABS electrical relays
are given in the Electrical Troubleshooting Manual
For location and identification of ABS electrical fuses.
See ELECTRICAL, Repair, Fuse Box - Interioror.
See ELECTRICAL, Repair, Fuse Box - Engine
Compartment

70BRAKES
2
FAULT DIAGNOSIS ABS FAULT AND REMEDY CHART
SYMPTOM POSSIBLE CAUSE CHECK REMEDY
ABS warning light on ABS electrical fault Check ABS electrical
circuit with Testbook
diagnostic equipmentChange component if
necessary
High sensor air gaps Push in sensors
Brake fluid warning light
ONFluid loss Check reservoir fluid level
and inspect system for
leaksRectify leakage, refill
reservoir
Reservoir fluid level
switch malfunctionCheck fluid level switch Change reservoir
cap/switch
Pedal travel increased,
foot pressure normalAir in system Rebleed brake system
Master cylinder
malfunctionChange master cylinder
Pedal can be moved
downwards under
constant pressureSeal leaking in master
cylinderInspect system for leaks Change master cylinder
Seal leaking in servo unit Change servo unit

BRAKES
1
REPAIR GENERAL BRAKE SERVICE PRACTICE
Brake fluid precautions
WARNING: Do not allow brake fluid to
come into contact with eyes or skin.
CAUTION: Brake fluid can damage
paintwork, if spilled wash off immediately
with plenty of clean water.
CAUTION: Use only correct grade of brake
fluid. If an assembly fluid is required use
ONLY brake fluid. Do NOT use mineral oil,
i.e. engine oil etc.
CAUTION: Thoroughly clean all brake
calipers, pipes and fittings before
commencing work on any part of the brake
system. Failure to do so could cause foreign
matter to enter the system and cause damage to
seals and pistons which will seriously impair the
efficiency of the brake system.
·To ensure the brake system efficiency is not
impaired the following warnings must be
adhered to :-
·DO NOT use any petroleum based cleaning
fluids or any proprietary fluids containing
petrol.
·DO NOT use brake fluid previously bled from
the system.
·DO NOT flush the brake system with any fluid
other than the recommended brake fluid.
The brake system should be drained and flushed
at the recommended service intervals.
Cover all electrical terminals carefully to make
absolutely certain that no fluid enters the
terminals and plugs.FLUID LEVEL CHECK / TOP UP
WARNING: Clean reservoir body and filler
cap before removing cap. Use only fluid
from a sealed container.
1.Park vehicle on level ground.
2.Check level is between 'MIN' and 'MAX' marks.
3.If level is below 'MIN' mark top up fluid level to
'MAX' mark on reservoir, using correct fluid.
See
LUBRICANTS, FLUIDS AND CAPACITIES,
Information, Recommended Lubricants and
Fluids
Do not fill reservoir above maximum line

70BRAKES
2
REPAIR BRAKE SYSTEM BLEED
Service repair no - 70.25.02
The following method for bleeding the hydraulic brake
system applies to both ABS and NON ABS vehicles.
Preparation
WARNING: Before bleeding the brake
system refer to general brake service
practice.
See General Brake Service
Practice
·During bleed procedure, brake fluid level
must not be allowed to fall below the MIN
level. Keep reservoir topped to the MAX level.
·To bleed the hydraulic circuits four bleed
nipples are provided, one at each caliper.
·There are two methods by which air can be
removed from the braking system:-
1. MANUAL BLEED PROCEDURE.
2. PRESSURE BLEED PROCEDURE.
Pressure bleed procedure
Purpose designed equipment for pressure filling and
bleeding of hydraulic systems may be used on Land
Rover vehicles. The equipment manufacturer's
instructions must be followed and the pressure must
not exceed 4.5 bar.
Manual bleed procedure
Equipment required
·Clean glass receptacle
·Bleed hose
·Wrench
·Approx 2 litres brake fluid.
See
LUBRICANTS, FLUIDS AND CAPACITIES,
Information, Recommended Lubricants and
Fluids
Master cylinder bleed
1.Disconnect battery negative lead.
2.Depress brake pedal fully and slowly 5 times.
3.Release the pedal and wait for ten seconds.
4.Air bubbles will rise into the reservoir during
these instructions.
5.Repeat instructions until a firm resistance is felt
at the pedal.
Complete circuit bleed
1.Disconnect battery negative lead.
2.Fit bleed hose to any caliper bleed screw.
3.Dip free end of bleed hose into brake fluid in
bleed bottle.
4.Open bleed screw of caliper.
5.Depress brake pedal fully several times until fluid
is clear of air bubbles.
6.Keeping pedal fully depressed, tighten bleed
screw, then release pedal.
7.Repeat the above procedure on remaining three
calipers.
8.Fit all bleed screw protection caps.
9.Check/top-up fluid level when bleeding is
complete.
See Fluid Level Check / Top-Up

BRAKES
3
REPAIR MASTER CYLINDER ABS
Service repair no - 70.30.08
- Lucas Girling - Type 25.4mm CV/CV
NOTE: This master cylinder is identified by
a 3rd plugged outlet port protruding down
from the master cylinder body.
Before starting repair refer to general brake service
practice.
See General Brake Service Practice
Remove
1.Disconnect battery negative lead.
2.Place a suitable container under master cylinder
to catch brake fluid from cylinder when brake
pipes are disconnected.
3.Thoroughly clean immediate area around all
outlet ports. Remove each brake pipe, sealing
each pipe and outlet port with suitable plugs to
prevent ingress of foreign matter.
4.Disconnect electrical plug from low fluid switch
on reservoir cap.
5.Remove two nuts securing master cylinder to
servo unit remove also spring and plain washers.
6.Detach master cylinder from servo, remove
reservoir cap and drain brake fluid into a
container.Refit
NOTE: Ensure O ring seal between master
cylinder and servo unit is fitted correctly.
7.Fit master cylinder to servo. Secure in position
with two nuts, plain and spring washers. Tighten
to
26 Nm.
8.Fit brake pipes to master cylinder. Tighten to
15Nm.
9.Top-up master cylinder with correct grade of
brake fluid.
See LUBRICANTS, FLUIDS AND
CAPACITIES, Information, Recommended
Lubricants and Fluids
10.Bleed system.See Brake System Bleed
11.Fit cap, reconnect electrical lead to fluid level
switch. Reconnect battery negative lead.

70BRAKES
4
REPAIR MASTER CYLINDER NON ABS
Service repair no - 70.30.08 - Remove
Service repair no - 70.30.09 - Repair
- Lucas Girling - Type 25.4mm AS/AS
WARNING: This master cylinder is not
suitable to be fitted on vehicles with
antilock brake systems.
Before starting repair refer to general brake service
practice.
See General Brake Service Practice
Remove
1.Disconnect battery negative lead.
2.Place a suitable container under master cylinder
to catch brake fluid from cylinder when brake
pipes are disconnected.
3.Thoroughly clean immediate area around all
outlet ports. Remove each brake pipe, sealing
each pipe and outlet port with suitable plugs to
prevent ingress of foreign matter.
4.Disconnect electrical plug from low fluid switch
on reservoir cap.
5.Remove two nuts securing master cylinder to
servo unit remove also spring and plain washers.
6.Detach master cylinder from servo, remove
reservoir cap and drain brake fluid into a suitable
container.Repair
7.Thoroughly clean master cylinder and inspect
outer surfaces for damage and condition, renew
complete assembly if necessary.
8.Using two soft jaws, clamp master cylinder
flange in a suitable vice. Remove water ingress
'O' ring seal from master cylinder to servo flange
and discard.
9.Grip outside of transfer housing with a suitable
pair of pliers, carefully pull, working pliers in a
backwards and forwards rocking motion to ease
housing off master cylinder. Discard housing and
vacuum seal.

BRAKES
5
REPAIR
10.Ease reservoir out of master cylinder taking care
to ensure that two outlet ports on bottom of
reservoir do not become damaged during this
process.
11.Withdraw two reservoir seals from master
cylinder inlet ports. Note for reassembly these
seals are different. Discard seals.
12.Remove retaining ring and 'O' ring seal from
machined outer surface of master cylinder.
Discard both seal and retaining ring.
13.Remove guide ring, which supports primary
plunger assembly, from mouth of master
cylinder. Place to one side for reassembly, note
this component is not part of service kit.
14.Pull primary plunger assembly out of master
cylinder.