Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual

Page: 301 / 413

Page 42
Light pressure on the foot pedal
CONDITION A
With very light pedal pressure, no servo assistance
is given. Fluid from the master cylinder enters the
servo unit hydraulic inlet and travels through
cavity V, past the push rod (13), through the hole
in the centre of the hydraulic piston (32) and out
of the servo unit hydraulic outlet to the wheel
cylinders. Chamber W is connected to the engine's
induction manifold and is therefore under de­
pression.
There is a through passage to chamber X via the
hollow stem of the control valve diaphragm (23),
and to chamber Y via the connecting pipe. Also
there is a through passage from chamber W to
chamber
Z.
Thus chambers W, X, Y and Z are all under
depression.
The air valve (20) remains on its seat due to the
influence of spring (21), and the air pressure on its
outer face.
Pressure on the foot pedal increased
(CONDITION B)
This results In an increase of hydraulic pressure on
the bottom face of the valve piston (11) which
causes it to move upwards taking with It the control
valve diaphragm (23), the stem of which butts
against the air valve (20) thus Isolating chambers
X and Y from the vacuum source. If the increase
in hydraulic pressure is great enough, the air valve
(20) is lifted off its seat causing air (at atmospheric
pressure) to be admitted to chamber X and thence
chamber Y.
The difference In pressure thus created across the
·booster diaphragm (9) Induces It to flex towards
the slave cylinder, taking with it the push rod (13).
The end of the push rod engages with the hydraulic
piston (32), blocking off the hole In its centre and
moving it down the slave cylinder bore, thereby
Increasing the hydraulic line pressure to the wheel
cylinders. There now exists, therefore, two
hydraulic line pressures:-
WSM. 12,t
Section
K (Brakes)
(a) The lower pressure line from the master
cylinder to chamber V.
(b) The high pressure line beyond hydraulic piston
(32) to the wheel cylinders.
Foot pedal held steady
(CONDITION C)
Since a pressure differential also exists across the
control valve dlaphram (23), there is a tendency for
this diaphragm to flex In the direction of the valve
piston (11). If the foot pedal is held steady, the
force tending to flex the control valve diaphragm
(23) downwards will balance the line pressure
tending to move the valve piston (11) upwards.
The control valve diaphragm (23) has at this point
already moved slightly downwards, permitting the
air valve (20) to return to its seat thus cutting off
any further supply of air.
The condition at this stage, therefore, ls that cavity
Z is under depression, whilst cavity Y ls cut off
from any further air supply. The booster diaphragm
(9) will remain stationary, the force due to the
pressure of air in cavity Y being balanced by the
spring (1-4) and fluid pressure behind hydraulic
piston (32) and no further servo assistance will be
given.
Pressure on the foot pedal further increased
(CONDITION B)
Increase in pressure from the master cylinder will
increase the force on the bottom of the valve piston
(11), re-opening the air valve (20) and increasing
the servo assistance in the manner previously
described.
When the opposing forces on the control valve
diaphragm (23) are once more in balance, the air
valve (20) will again close on its seat. It will be
apparent, therefore, that the control valve dia­
phragm (23) acts as a balancing device so that for a
given foot pedal pressure, a certain amount of
servo assistance is supplied, which Is proportional
to the pedal pressure.
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