MG MGB User Manual

THE BRAKING SYSTEM
M
My
’
M0692A
l
Fig. M.9
A
schematic diagram
showing the
principle of
operations
and
the main
components
of the
vacuum servo
unit. The shaded
area
represents
brake
fluid
Section M.12
SERVO UNIT
(Lockheed
Type
6)
Operation
Refer
to
Fig. M9.
The
vacuum-operated
servo
unit consists
of
three
main
components,
namely the
vacuum
cylinder
’
(1),
the
air
valve
assembly
(2),
and the slave cylinder
(3)
which
is
connected
in
the
hydraulic
circuit
between the
main
master
cylinder and the
wheel cylinders. Under light
braking ■uid
is
allowed
to
pass
directly
to
the wheel
cylinders
via
the hollow
centre
of the slave
piston
(4)
and
no
braking
assistance is
obtained; ■uid
pressure
acting
on
the
air
valve
piston
(5)
closes
the diaphragm
(6),
thus
separating
the chamber behind the
main
servo
diaphragm
(7)
from the
one
in
front. Under
heavier braking, further
movement
of the
air
valve
piston
opens
the
air
valve and
allows
air
to enter
the
chamber behind the
main
diaphragm,
destroying
the
vacuum.
The central rod
(9)
is
thus
pushed
to
the left, sealing the hollow
centre
of the slave
piston
and
pushing
it
down
its
bore,
so
increasing
the ■uid
pressure
at
the wheel cylinders. When
the brake pedal
is
released,
the
pressure
beneath the
air
valve
piston is
destroyed,
the
diaphragm
(6)
re-opens
and the
air
valve closes. Via the
non-return
valve
(10),
a
suspended
vacuum
is
recreated
around the
main
diaphragm. Under the
action
of the
spring
(11),
the diaphragm
and push-rod, and
thus the slave
piston,
are
returned
to
their
original
positions,
and the
pressure
in
the wheel cylinders
is
lost.
Removing
1. Disconnect
the battery.
2. Remove
the
two
captive
screws
securing
the base
of
the
servo
cylinder
bracket
(LHD
cars),
one screw
(RHD cars).
3.
Remove
the bolt
securing
the
vacuum
hose clip
to
the
servo
cylinder
bracket.
80619
MGB.
Issue1.
4. Disconnect the
master
cylinder
feed
outlet hydraulic
connections
at
the
servo.
Ensure
the feed
pipe
from
the
master
cylinder
is
blanked
to
prevent
■uid
loss.
R.H.D. CARS.
Remove
the three
nuts
and
spring
washers
retaining
the
servo
to
the
rear
bracket.
Remove the
servo
unit
and
extract
the
vacuum
hose
connection
from
the
servo.
L.H.D. CARS.
From
inside the
car,
remove
the
bulkhead
grommet
and
remove
the
three
nuts
and
spring
washers
retaining
the
servo
to
the
rear
bracket.
Remove the
servo
unit
and
extract
the
vacuum
hose
connection
from the
servo.
Dismantling
AIR VALVE
ASSEMBLY
(FIG.
M.
l
0)
5.
Grip the slave cylinder
in
a
soft jawed
vice
with the
air
valve
uppermost
and
disconnect the rubber
pipe
from
the
connection
on
the
end
cover.
6. Remove the
screws
securing
the plastic
air
valve
cover
to
the valve housing, lift off the
cover
complete with
the
air
valve
sub-assembly.
Suspect
functioning of the
air
valve
must
be
remedied by
fitting
a
replacement
air
valve
cover
assembly
comprising
cover,
filter,
and
air
valve
as an
assembled
part
of the relevant
repair
kit.
7. Remove
the rubber diaphragm and
its
plastic
support
to
obtain
access
to
valve housing
securing
screws.
Remove
the
three
screws
and take
off
the
housing
and
joint
washer.
8. Seal
one
of the
slave
cylinder ■uid
ports
with
a
■nger,
apply
a
low-pressure air-line
to
the
remaining
port
and
blow the
air
control valve
piston
from
its
bore.
Remove
the rubber
cup
from the valve
piston.
Fig.
M.1U
The
air valve
assembly
components
and piston
1.
Domed
cover
for ■lter
7. Valve housing
securing
2. Air filter.
1
screws.
3.
Air valve
cover
securing
8.
Valve housing.
,'~i_
“
screws.
9.
Jo'int
washer.
I
.
"
4.
Air valve
cover.
10. Piston.
'
5.
Diaphragm.
11.
Piston
cup.
["5
ii"
6. Diaphragm
support.
12. Slave
cylinder.

MG MGB Specifications

General

Brand	MG
Model	MGB
Category	Automobile
Language	English