Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual, Page: 298 / 413

WSM.12-4
Section K (Brakes) Page 39
THE LOCKHEED VACUUM SERVO UNIT
GENERAL
The purpose of the vacuum servo unit is to lower
the pedal pressure r�quired to produce a given
braking effect. This is achieved, essentially, by
allowing fluid pressure from the master cylinder to
operate· a valve situated In the servo unit, which
admits air to one side of a diaphragm contained
within a· vacuum cylinder (the vacuum being
derived from the engine's inlet manifold).
The difference in pressure thus created across the
diaphragm Induces It to flex towards the low
pressure side, taking with it a push rod which
boosts the line pressure of the brake fluid going
to the brakes. This operation Is described In more
detail under the heading "Principles of Operation".
The design of the servo unit is such that, If for any
reason the servo fails to function, there remains· an
unrestricted passage for brake fluid to pass from
master cylinder to wheel cylinder, so that normal
unassisted braking is still available.
MAINTENANCE
Servo Air Filter
The filter in the air valve cover of the servo unit
should be cleaned at regular intervals.
The procedure Is as follows:-
I. With the ignition switched off, pump the foot
brake to destroy any vacuum in the servo unit.
2. Remove the air vafve cover assembly (item 19,
Fig. 22) by withdrawing the five securing
screws.
3. Lift the air valve (item 20, Fig. 23) off its seat
and blow compressed air at low pressure into
the filter chamber. Do not lubricate the filter
or attempt to remove it from the air valve
cover.
4. When renewing the filter, fit a complete air
valve cover.
DESCRIPTION
The unit is installed in the hydraulic braking system
between the master cylinder and the wheel cylin­
ders; that Is to say, the outlet pipe from the master
cylinder Is connected to the inlet of the servo unit
and the servo unit outlet Is connected by pipes to
the brake wheel cylinders.
The servo unit may be considered in three sections:
I. The Slave Cylinder from which pressure Is
applied to the brakes.
2. The Vacuum Cylinder which supplies the
force required to operate the slave cylinder.
3. The Control Valve Gear which regulates the
actions of the other two sections.
Reference should be made to Fig. 22 for details of
the parts which comprise these three sections.
PRINCIPLES OF OPERATION
The operation is shown diagrammatically in Fig. 24.
The reference numbers used correspond with those
used In the exploded view (Fig. 22) and sectional
view (Fig. 23).