Discovery 2. Manual - part 629

 

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Discovery 2. Manual - part 629

 

 

BRAKES

DESCRIPTION AND OPERATION

70-7

Section through brake servo assembly - brakes off

Non return valve
Housing
Diaphragms
Minor diaphragm
Tie bolt
Valve body
Valve
Valve spring
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

Partial vacuum

Ambient

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

Non return valve
Housing
Diaphragms
Minor diaphragm
Tie bolt
Valve body
Valve
Valve spring
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

Partial vacuum

Ambient

Servo

BRAKES

70-10

DESCRIPTION AND OPERATION

Vacuum pump (diesel models only)

Alternator
Vacuum pump
Brake servo assembly vacuum connection
Oil feed pipe
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.

 

 

 

 

 

 

 

Content   ..  627  628  629  630   ..