Discovery 2. Manual — part 208

BRAKES

DESCRIPTION AND OPERATION

70-7

Section through brake servo assembly - brakes off

1 Non return valve
2 Housing
3 Diaphragms
4 Minor diaphragm
5 Tie bolt
6 Valve body
7 Valve
8 Valve spring
9 Input rod spring

10 Input push rod
11 Filter
12 Gaiter

13 Piston
14 Ratio disc
15 Reaction disc
16 Central plate
17 Master cylinder
18 Return spring
19 Guide tube
20 Output push rod
21 Bush
22 Air pressures

a Partial vacuum

b Ambient

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

1 Non return valve
2 Housing
3 Diaphragms
4 Minor diaphragm
5 Tie bolt
6 Valve body
7 Valve
8 Valve spring
9 Input rod spring

10 Input push rod
11 Filter
12 Gaiter
13 Piston

14 Ratio disc
15 Reaction disc
16 Central plate
17 Master cylinder
18 Return spring
19 Guide tube
20 Output push rod
21 Bush
22 Air pressures

a Partial vacuum

b Ambient

c Servo

BRAKES

70-10

DESCRIPTION AND OPERATION

Vacuum pump (diesel models only)

1 Alternator
2 Vacuum pump
3 Brake servo assembly vacuum connection

4 Oil feed pipe
5 Oil 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.