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Max. part ia
I
vacuum
_
Atmosph
e
ric
pressure
Fig
. 5-72. Full
brake applicatio
n
VOLVO
103203
eq uivalent
vacuum
exists
on both
sides of
the
dia
-
phragm which, together with the
guide
housing,
is
held
pressed
to the right
end
position of the
dia-
phragm spring.
When the brake
pedal
is
depressed,
the
rear
thru
st
rod
and
valve
piston
are moved to the left
(for-
wards).
The va lve
spring
causes
the valve plate
to
move also until it reaches the
seat
in the guide
housing. This c10ses the
connection
between the front
and rear side of the diaphragm. When the piston
continues moving,
its
movements
are transferred via
the reaction disc and front thrust rod to the master
cylinder.
When the sea t
of
the
valve
piston leaves
the
plate, the
con
nection
between
th
e
rear
side
and the
centre of the valve
section
is opened. Air from atmos-
pheric pressure can then flow
in
behind the dia
phragm. When there is partiai
vacuum
on the front
side of the diaphragm, it is moved, and also the
guide housing, forwards.
In thi
s
way, the force
app-
lied
to the front thrust rod
is
increased. The parts of
the servo cylinder are
in
the position
shown in Fig.
5-72 when the pedal pressure provides maximum
servo effect.
If
the pedal
pressure
IS
less
than
that mentioned
above, the same procedure takes place
in the
be-
gmnmg.
During
brake
application,
"the
hydraulic
pressure
in the
master cylinder
increases
and also
the
counterpressure
on the
front thrust rod. The pressure
of the
guide
housing is
transmitted to the thrust rod
through
the
outer part
of the
reaction disc. Because
the disc
is
made of rubber,
its
periphery contracts
while its
centre
tends to
expand, see Fig. 5-73. This
causes the
guide
housing
to be moved further
for-
wards than the valve piston
and results
in
the seat of
the piston reaching the
valve
shutting off the
a
ir
5:
30
supp ly. The press ure behind the diaphragm remams
constant and is t hus unab
le
to overcome the hydrau-
li
c counterpress ure in t he master cy
lin
der. The mov-
able parts of the se rvo cylind er, therefore,
re
main in
this position, and con stant b raking is obtained
as
long
as the same pressure is maintained on the brake
pedal.
If pressure
on
the
pedal
IS
increased,
the pressure
of
the
valve piston
on
the
reaction disc centre will
be
greater,
this
causing a certa in displacement for-
wards of the piston. When this happens, the valve
leaves the seat of the piston, more air can flow
in
and greater brake application is obtained
until
the
new equalizing position is attained.
If
the pressure on the pedal is reduced, the reaction
disc centre can be thrust out still further, and thi
s
causes the valve piston to lift the valve from the
seat
in
the guide housing. The
spaces
on both sides
of the diaphragm are thereby connected with each
other, equal pressure arises, the guide housing is
moved backwards by the spring pressure and there
is
a reduction in the brake application
.
This
proce-
dure
a lso
reduces
the contraction of the reaction disc
periphery,
so
that the valve
piston can return to
the position shown
in Fig. 5-73 and
the new
equaliz-
ing
position is
rea
ched.
If
the
brake
pedal is released
fully,
all
the
parts of the
servo cylinder are returned
to the
rest position and the brakes
are
released.
Should any
fault o ccur
with the vacuum supply, brake
application can
still take
place due to the fact that
the
servo cylinder functions as an
extended
thrust
rod.
As
no servo
effect
is then obtained, greater pressure
on the pedal
is of
course required.
Max. partiaI
vacuum
PartiaI
vacuum
_
Atmospheric
pressure
YS~
Fig
. 5-73.
PartiaI brake appl ication
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