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8-6
'
S E R
VICE RECOMMENDATIONS
BRAKES
(2
50)
draulic
system, no
braking action can take pla ce
unti l all of t
he shoes are in
contact
with
the
drums;
t
here for e,
the system is
self-equalizing.
As pr essure on
brake pedal is inc
reased,
greater
hydraulic pressur e is built up
within
the
mas-
t
er cylinder
ba r r el and
whe el cylinders,
and
consequent ly greater fo
rce is
exer ted against
all
brake shoes.
As t
he
brake
shoes
come in contact with the
dr ums,
on
forwar d
motion, the ener gy of the
turnin g drums is imparted to
the
primar y shoes
which
then
actuate
the
secondar y
shoes so that
all shoes are
f
orced against the
drums
wit h
greater pressure
t
han
that provided by the
hy-
draulic
syst em. This
self-energizing pr inciple
gi
ves
maximum
braking
power
at
t
he wheels
with relatively
light
pressure on
t
he br
ake
pedal.
When the
car
is
being slowed
down
by the
br ak es,
a t
ransfer of weight
takes
place f
rom
t
he r
ear
to t
he
front end
of
car.
The quicker
the
stop
the gr eat er
t
he transfer
of
weight
.
This
t
ransfer
of
weight t
o the
front wh
eels
tends
to keep
the front wheels turning, conse-
quently
more braking power
is
re
quired
at
front
wheels
in order to
equalize
t
he
braking effect
at front
and re
a r wheels.
To provide greater
braking
power
at th
e front
wheels,
t
he front
wheel
cylinders a
r e larger in
diamet er
than
the
r
ear wheel
cylinders. Braking power
at front
wheels is
approximately 53
o/c
and at
rear
wheels approximately 47
O/C
.
,
c. Slow
Release
of Br a k e s
As the brake pedal is r
eleas ed the pedal
and
push rod are retracted by
pedal
return spr ing,
thereby
relieving
pressure on
mas ter cylinder
piston
and consequently r
elieving
pressure in
the master
cylinder
ba
r r el,
brake pipes, and
wheel cylinders.
The
master
cylinder
spring
(R)
forces the
piston (0) and piston
primary
cup
(Q)
to fol-
low the
retracting
push rod (K)
.
See figure
8-5.
At the same
t
ime,
the brake
sho
e
sp
ri ngs pull
the shoes clear of brake
drums thereby
forcing
wheel
cylinder
pistons
in
ward,
which forces
fluid out of cylinders
back
to
the
master cyl-
ind
er.
Returning fluid pushes check
valve (T )
away
f
r om the
rubber
washer (U)
on
head nut
(J)
as it
enters
master
cylinder
barrel (S)
,
how-
ever, the
master
cylinder
spring
(R) maintains
sufficient pressure
on
t
he
check
valve
to
provide
6 to
8 pounds static
pressure in
brake
pipes
and
wheel cylin ders after
brakes a
re
fully re
leased.
When
brakes
are full
y released,
the
master
cylinder piston
must
bear
against
the
stop
plate
(L
) so that the
comp
ens ating
port (H)
will not be
cover ed
by
t
he
piston
primary cup
(
Q ).
Th
e
open
comp ensat ing
port
permits
a
flow
of
fluid
from
reservoir into the
cylinder
ba
rrel
in
t
he
event
that fluid was
lost by
leak-
age
during brake application.
d. Q
uick Release of Brakes
The
action
in
qu
ick release is generall y the
same as in
the
slow r
elease
described above,
except for a
compensating
action in the master
cylinder .
The piston (D) arid primary cup
(Q)
return
to
the
"released"
position much
fas t er than
the
fluid in
brake
pip
es
returns t
o master
cylinder
barrel
(S
) .
A
momentary
vacuum is
created
in
the
space
between primary cup and head nut
which causes additional fluid to flow into
and
fill this space
through
the bleeder holes (P) in
piston and past the
lip
of
primary
cup. This
additional fluid is supplied through the breather
port
(G). When the piston
r
eache s the st op
plate (L) and the
compensating
port (H) is
un
covered
by the primary cup, surplus fluid re-
t
urns
to reservoir through the compensating
port.
e. Action
During
Bleeding O
peration
When brake pedal is
depressed
during the
bleeding
operat ion
(par.
8-9)
fluid
is
forced
out
of
master
cylinder through
the
check valve
(T)
,
through brake pipes to wheel cylinder be-
in
g bled.
Sinc
e
no pressure is built up in wheel
cylinder ther e is no
return
of
fluid
to master
cylinder when brake pedal is released. The mas-
ter
cylinder
barrel (S) is replenished with fluid
wh
ich
flows from
reservoir
through breather
port
(G),
t
hr ough bleeder holes in piston (P),
and
pa
st
edg es of
primary
cup
(Q).
See
figure
8-5.
8-4 SERVICE RECOMMENDATIONS
a.
Bre a k - In of New
Brake Shoe Linings
Under
no cir cumstances should
the
brakes
be severely applied
on
a new car
or immedi-
ately after installation of new brake shoes or
lini
ngs.
Seve
re
applications ma y permanen tl y
injure new br ak e
linings and
may scor e brake
drums.
When lini ngs
are new t
hey must
be
gi
ven moderate
use
for
several
days until
ni
cely
burnished.
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