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BRAKE VALVE
A
brake valve
is connected
to
each of
the rear wheel
brake
lin es,
see
Fig
.
5-6.
When
the ingoing brake
pressure exceeds 34 kg /cm
2
(484 Ib/sq.in
.)
a reduction
takes
place
in the
va!ve.
The
more powerful the
pedal pressure, the greater will be the reduction and
th ereby
the larger the difference
between
the hydrau lic
pressure
in
the front whee
l
and rear wheel cylinders.
This resu lts in a
suitab
le distribution of braking force
between both pairs of wheels. The construction of the
brake valve is shown
is
Fig. 5-41 and its function is
the following.
When the
footbrakes
are applied, the pressure from
the master
cylinder
is transmitted via the
co
nnection
(7, Fig.
5-41 ). The pressure
then proceeds through the
cylinder
(6),
the
counterbore, past
th
e
valve
s
(17)
and
(4)
to cylinder
(3)
and
then
on
through
connection
(19)
to the
rear
wheel cy
lin
ders, see Fig. 5-42. The
hydrauli
c
pressure
per unit
surface
is
equal
on the
different parts of the
piston
(21
), but since its
pres-
sure surface
is larger in cylinder
(3)
than in
cy linder
(6), the
force
developed will move the piston to the
right
of
the
figure.
However, this
is
counteracted by
the pressure from the
springs (10).
When the hydrau lic pressure approaches 34 kg /cm
2
(484 Ib/sq
.in .)
the spring pressure
is
overcome a nd the
piston (21
)
is
moved
to the right. By means of
pres-
sure from the sma
ll
er spring
(5), the valve
(4) shu
ts
off the co
nn
ection between
the two
cylinders and
forms two
sepa rate
systems,
one for
th
e
front wheels
and one for the
rear wheels.
With
co
ntinu
ed
in
crease
in
press ure
in the ma
ste
r
cylin
der and
front wheel cylinders,
th
e
hydraulic for
ce
Fig.
5-41.
Brake
valve,
construction
1. Plug
12. Adjusting screw
2.
O-ring
13.
Locknut
3. Cylinder
14. Spring housing
4.
Valve
15. Retainer
5.
Valve spring
16. Screw
6.
Cylinder
17.
Equalizing valve
7.
Connection to 18. O-ring
master cylinder 19. Connection to rear
8. Piston gasket
wheel brake
cylinders
9. Brocket
20. Housi ng
10.
Spri ng
21.
Piston
11. Retainer
22.
Valve housing
VO LVO
101 BBJ
Pressure
below
34
kg
/c
m
1
(484 Ib/sq.
in.
)
Fig. 5-42.
Brake appl ication
In
cy
linder
(6) moves the
piston
to the left
so
that
the valve rod
comes
up
against
its
stop and
opens
the valve thi
s
causing
the
pressure
in
cylinder
(3)
to
increase.
Due
to the larger pressure
surface
in this
cylinder,
th
e
piston
is moved to the right
aga
in
and
the valve c1oses. In this way, the piston
ass
umes
a
position of balance and the outgoing pressure from
the
brake valve will be lower than the
ingoing
pressure, see Fig. 5-42. The difference
in
these
pres-
sures
is
determined
by th
e
different areas and
sprin
g
tension.
When the brake
peda
l
is
released, the pressure
in th
e
cylind
er
(6)
falls.
The
piston
(21)
is moved
to
the
ri
g ht
by
sprin
g
(10).
When the pressure on the
right-
ha
nd sid
e
of
the va lve
(4)
falls
so
mu ch th
at
th
e
hydraulic pressure on the
left-
han
d
sid
e e
nabl
es
th
e
valves to
be
actuated,
the co
nn
ectio
n
between both
the
cylinders is
opened
again.
As the pressure falls,
sp
ring (10)
presses the
left
piston back
to
its
original
position
where
th
e
valve
is
held in
the
open position
by
mecha nica
l
mea
ns,
see Fig.
5-41. The
equalizing
valve
(17)
is
fitted
with controi chann
e ls
which
e
nsure
an
even
flow of pressure through the valve.
VOLVO
101
BB4
_
Reduced pressure
_
Pressure above 34 kg /cm
2
(484Ibjsq .in .)
Fig.
5-43.
Reducing action
5
: 19
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