Defender. Manual — part 34
Brake System - General Information - Brake System
Description and Operation
Master cylinder description
The mechanical components of the hydraulic braking system consists of four piston caliper disc brakes at the front and
two piston caliper disc brakes at the rear.
Vented front brake discs are fitted as standard on 110/130 models, while 90 models have solid discs. However, on 90
models with a heavy duty chassis, vented front discs may also be fitted.
A cable controlled parking brake operates a single drum brake mounted on the output shaft of the transfer gearbox and
is completely independent of the main braking system.
The basic hydraulic system involves 2 separate and independent primary and secondary circuits which permits a degree
of braking should a fault occur in one of the circuits. The primary circuit operates the rear brake calipers and the
secondary circuit the front brake calipers.
Master cylinder components
Item
Part Number
Description
1
-
Secondary plunger
2
-
Secondary spring
3
-
Recuperation seal
4
-
Primary spring
5
-
Recuperation seal
6
-
Primary plunger
Master cylinder operation
A tandem master cylinder, which is assisted by a light weight, short, compact servo, is fed by a divided fluid reservoir.
The rear section supplies fluid for the primary circuit and the front section the secondary circuit.
When the brakes are off, the fluid can move unrestricted between the dual line system and the separate reservoirs in the
fluid supply tank.
When the footbrake is applied, the primary plunger assembly moves up the cylinder bore and the pressure created acts
in conjunction with the primary spring to overcome the secondary springs, thus moving the secondary plunger assembly
up the bore. At the same time initial movement of both plungers takes the recuperating seals past the cut-off holes in
the cylinder chambers 'A' and 'C', see J6321, and applies pressure to the fluid in those chambers, which is directed to
the respective circuits.
The fluid in chambers 'B' and 'D' is unaffected by movement of the plungers and can move unrestricted between the
separate chambers and respective reservoirs in the fluid supply tank, both before and during brake application. When
the brakes are released, the plunger assemblies, aided by the return springs are retracted faster than the fluid; this
creates a depression between the fluid in chambers 'A' and 'C' and the recuperation seals.
The recuperation seals momentarily collapse allowing fluid in chambers 'B' and 'D' to flow through the holes in the
plungers, over the collapsed seals and into chambers 'A' and 'C' respectively. The movement of fluid from one set of
chambers to the other, is compensated for by fluid from the separate reservoirs in the supply tank moving through the
feed holes in the cylinder. Conversely, the final return movement of the plunger assemblies causes the extra fluid in
chambers 'A' and 'C' to move through the cut off holes into the fluid reservoir.
The servo unit provides controlled power assistance to the brake pedal when pressure is applied. Power is obtained from
a vacuum pump located on the RH side of the engine cylinder block. The vacuum is applied to both sides of a flexing
diaphragm, and by admitting atmospheric pressure to the rear diaphragm, assistance is obtained. The servo unit is
mounted between the brake pedal and master cylinder and is linked to these by push rods. Should a vacuum failure
occur, the two push rods will act as a single rod allowing the brakes to function in the normal way, although more effort
will be required to operate the brake pedal.
Master cylinder overhaul components
Item
Part Number
Description
1
-
Water ingress seal
2
-
Transfer housing
3
-
Vacuum seal
4
-
Guide ring
5
-
Retaining ring
6
-
O-ring seal
7
-
Primary plunger assembly
8
-
'L' seal
9
-
Secondary plunger
10
-
Washer
11
-
Recuperating seal (primary cup)
12
-
Seal retainer
13
-
Springs
14
-
Swirl tube
15
-
Master cylinder body
16
-
Reservoir seals
17
-
Reservoir
18
-
Low fluid level switch and cap
Hydraulic system
The brake system should be drained and flushed at the recommended service intervals.
A brake fluid loss switch is fitted to the master cylinder reservoir filler cap. The switch is wired to a warning light on the
vehicle fascia and will illuminate as a bulb check when the ignition is switched on and extinguishes when the engine is
running and the handbrake is released. A hydraulic failure in the system will result in fluid loss, causing the warning
light to illuminate.
On 90 models a pressure reducing valve (PRV), fitted to the RH bulkhead in the engine compartment, maintains the
braking balance, see J6322. Pressure to the rear calipers is regulated by the PRV, this valve is of the failure by-pass
type, allowing full system pressure to the rear brake calipers in the event of a front (secondary) circuit failure.
90 Models
110/130 Models
• NOTE: In some countries, a pressure reducing valve may be fitted to 110 models to conform to legal requirements.
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