5B-6
1968
PONTIAC
SERVICE
MANUAL
4. Place master cylinder or Tool J-22805-01 in
vise with front housing up.
5. Place diaphragm return spring In front housing,
small
end down.
6. Apply silicone grease or talcum powder to all
surfaces of outer bead of diaphragm that bear against
front and
rear
housings.
7. Place
rear
housing assembly over diaphragm
return
spring and, using
tool
J
9504
press down
firmly
on
rear
housing, guiding push rod into front
housing seal making certain scribe marks
will
align
when housings are locked
together.
Rotate
tool
counter-clockwise to lock the two housings. Vacuum
may be applied to the check valve to help draw the
housings
together.
CAUTION:
Do not
release
pressure
on
rear
hous-
ing
until
the
housings
are
fully
locked.
NOTE;
Be
sure
diaphragm
is not
pinched
during
assembly.
ASSEMBLE
MASTER
CYLINDER
TO
POWER
UNIT
Remove
master
cylinder and power brake assem-
bly from vise and remove old master cylinder
assembly or Tool J-22805-01.
NOTE;
Before
reassembling
master
cylinder
to
power
section,
the
distance
from
the
outer
end of
the
push
rod to the
master
cylinder
must be
measured
as
explained
under
PUSH
ROD
ADJUST-
MENT
below.
PUSH
ROD
ADJUSTMENT
The
push rod is designed with a self-locking ad-
justment screw to provide the correct relationship
between
the vacuum power piston and master cylin-
der piston. The adjustment screw is set to the cor-
rect height at the time of original assembly of the
power unit. Under normal service conditions the
adjustment screw
does
not require any further at-
tention providing the push rod assembly remains in
the original unit.
Whether a new push rod is used or the push rod
assembly is transferred to a unit other than the
original
one, the distance from the end of the adjust-
ment screw to the mounting face of the power cylin-
der should be rechecked either with a micrometer
depth
gauge
to a dimension of 1.225 to
1.210"
or
with height
gauge
J 22644. Place
gauge
over the
push rod on the front housing. Cutout portion of the
gauge
should never be lower than the adjustment
screw end of the push rod and the gap
between
the
cutout and
edge
of the push rod end should never
exceed
.010". (Fig. 5B-7)
To
adjust push rod, grip splined area of push rod
with pliers, being careful not to scratch machined
shaft. (DO NOT
REMOVE
PUSH
ROD
FROM
POWER
CYLINDER
SINCE
REACTION
DISC
MIGHT
BE
PULLED
OUT OF
DIAPHRAGM
PLATE
AND
FALL
INTO
FRONT
VACUUM
CHAMBER.)
Use a 5/16"
wrench
to turn adjusting nut in to shorten or out
to lengthen push rod.
When
push rod adjustment is correct, assemble
master cylinder assembly to the power unit vacuum
cylinder
at two studs. Secure with two nuts and lock
washers tightening to 25 lb. ft. torque.
After
assembly of the master cylinder to the power
unit, the primary cups of the master cylinder must
clear
the compensating
hole
when the unit is in the
released position.
This
can be checked by partially
filling the reservoir, and then stroking the power
unit. If air bubbles appear or fluid spurts, the com-
pensating ports are clear. If the primary cups over-
lap the compensating ports, there
will
be no flow of
air
or fluid through the compensating port when
stroked. If this condition exists, the adjusting screw
should be turned into the push rod a slight amount,
or until the compensating port is open.
Failure
to
clear
the compensating port in the released position
traps fluid in the hydraulic lines and wheel cylinders
and
causes brake drag when the fluid warms up.
If
compensating port is blocked, fluid from pres-
sure bleeder
will
flow thru bypass main metering
port behind primary cup and then thru
holes
in
piston, around lip of primary cup to wheel cylinders.
TORQUE
SPECIFICATIONS
Piston Stop Screw
............
40 lb. in.
Power cylinder housing to
master cylinder nuts 25 lb. ft.
Rear
housing to dash nuts 25 lb. ft.
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