Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual

Page: 290 / 413

WSM.124
Section K (Brakes) Page 31
GIRLING VACUUM SERVO UNIT
The Girling Vacuum Servo Unit Is Installed In the
brake hydraulic system between the master cylin­
der and the brake assemblies on the roadwheels
with master cylinder outlet pipe connected to the
vacuum servo unit and the servo hydraulic outlet
is connected to the brake assemblies. The force
required to augment the drivers effort is obtained
by admitting atmospheric pressure to a vacuum
cylinder containing a piston.
The pressure difference thus created across the
piston produces a thrust load which Is used to
increase the, hydraulic pressure available at the
brake assemblies.
In the Girling Vacuum Servo Unit, the piston In
the vacuum cylinder is normally subjected to
vacuum on both sides and this prlnclple Is known
as "Suspended Vacuum System". The vacuum Is
obtained from the Inlet manifold of the engine
and therefore servo assistance Is only available
while the engine is running.
Between the engine Inlet manifold and the vacuum
servo unit Is a non-return valve which prevents
air or petrol fumes entering the servo unit.
Operation (Fig, 20)
When air, at atmospheric pressure, Is admitted
to the vacuum cylinder (19) by a composite con­
trol valve the piston (5) drives the piston rod
(18) and the output piston (25) down the hydraulic
output cylinder (28) providing a considerable In­
crease of hydraulic pressure to the brake assemblies
on the roadwheels.
The control valve, operated by hydraulic pressure
from the master cylinder, exercises a precise con­
trol over the pressure Increase and the brakes
are applied exactly In proportion to the pressure
applied to the foot pedal.
When the servo unit Is at rest with no pressure
In the hydraulic system, the vacuum valve (22) of
the control valve Is open and permits vacuum
from the Inlet manifold to temporarily communi­
cate with the second side of the piston (5) In the
vacuum cylinder (19) thus vacuum on both sides
of the piston (5) Is equal.
When pressure is applied to the brake pedal, hy­
draulic pressure Is exerted throughout the braking
system and equally on both ends of the valve con­
trol piston (10 & 13). As one end of this control
piston Is larger than the other, an equal pressure
per square Inch on both Its ends causes a propor­
tionally greater thrust to be exerted on the larger
end moving the control piston and results In the
"Tee" shaped lever (11) opening the air valve (31)
to the atmosphere and closing the vacuum valve
(22). Air Is admitted to the second side of the
piston (5) destroying the temporary vacuum, driv­
ing the piston (5) forward. The piston rod (18)
seals the centre hole In the output piston (25) and
continued movement applies additional pressure to
the hydraulic pressure proceeding to the brake
assemblies and also to the small end of the valve
control piston (13).
The movement of the output piston (25) continues
until the thrust on the small end of the valve con­
trol piston (13) by the higher pressure to the brake
assemblies, overcomes the thrust on the large end
of the control piston (10), by the lower pressure
from the master cylinder. The valve control piston
(10 & 13) is thus moved back and closes the air
valve (31); at this point both valves are closed and
the brakes held on.
When the brake pedal is released the lower pres­
sure from the master cylinder Is reduced at the
large end of the valve control piston ( I 0) which
moves back towards Its rest position causing the
"Tee" shaped lever (11) to open the vacuum valve
(22) drawing the air from the second side of the
piston (5) In the vacuum cylinder ( 19), the piston
(5) returns to Its rest position and with It the out­
put piston (25) relieving the hydraulic pressure In
the brake assemblies. The piston rod ( 18) Is with­
drawn from the centre hole In the output piston