300Tdi Defender Brake Operation
70
BRAKES
2
DESCRIPTION AND OPERATION
OPERATION
Master cylinder
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 fliud 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 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 atmosheric 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.
Hydraulic system
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.
NOTE: In some countries, a pressure
reducing valve may be fitted to 110 models
to conform to legal requirements.
