Buick LeSabre 1965 Service Manual page 530

9 - 8 OPERATION
BRAKES
d. Self Adjusting Brake
The self adjusting brake mecha-
nism consists of an actuator,
actuator pivot, actuator return
spring, override spring and an
actuating link. See Figure 9-13.
The self-adjusting brake mechan-
ism is mounted on the secondary
shoes and operates only when the
brakes are applied while the car
is moving in a rearward direc-
tion and only when the secondary
shoe moves a predetermined dis-
tance toward the brake drum.
When the car is moved in a rear-
ward direction and the brakes are
applied, friction between the pri-
mary shoe and the drum forces
the primary shoe against the an-
chor pin. Hydraulic pressure in
the wheel cylinder forces the
upper end of the secondary shoe
away from the anchor pin, the
upper end of the actuator is pre-
vented from moving by the actu-
ating link. This will cause the
actuator to pivot on the secondary
shoe forcing the actuator lever
against the adjusting screw star
wheel. If the brake linings are
worn enough to allow the second-
ary shoe to move the predeter-
mined distance, the actuator will
turn the adjusting screw one
tooth. If the secondary shoe does
not move the predetermined dis-
tance, movement of the actuator
will not be great enough to rotate
the adjusting screw.
When the brakes are released,
the actuator return spring will
return the actuator into adjusting
position on the adjusting screw.
9 - 3 OPERATION OF
HYDRAULIC SERVICE
BRAKES
NOTE: See Sections 9-D and 9-E
for power brakes.
When the brakes are fully re-
leased, the master cylinder piston
is held against the stop plate and
the primary cup is held just clear
of the compensating port by the
master cylinder spring, which
also holds the check valve against
its seat on the valve seat washer.
The pressure chamber is filled
with fluid at atmospheric pres-
sure due to the open compensating
port and reservoir rubber dia-
phragm. All pipes and wheel cyl-
inders are filled with fluid under
a "static" pressure of 8-16
pounds per square inch, which
helps to hold the lips of the wheel
cylinder cups in firm contact with
cylinder walls to prevent loss of
fluid or entrance of air. See Fig-
ure 9-9, View A.
When the brake pedal is de-
pressed to apply the brakes, the
push rod forces the master cyl-
inder piston and primary cup for-
ward. As this movement starts,
the lip of the primary cup covers
the compensating port to prevent
escape of fluid into the reservoir.
Continued movement of the piston
builds pressure in the pressure
chamber and fluid is then forced
through holes in the check valve
and out into the pipes leading to
all wheel cylinders. Fluid forced
into the wheel cylinders between
the pistons and cups causes the
pistons and connecting links to
move outward and force the brake
shoes into contact with the drums.
See Figure 9-9, View B.
Movement of all brake shoes into
contact with drums is accomp-
lished with very light pedal pres-
sure. Since pressure is equal in
all parts of the hydraulic system,
effective braking pressure cannot
be applied to any one drum until
all of the shoes are in contact
with their respective drums;
therefore the system is self-
equalizing. After all shoes are
contacting the drums, further
force on brake pedal builds up
additional pressure in the hydrau-
lic system, thereby increasing the
pressure of shoes against drums.
On rapid stops some car weight is
transferred from the rear to the
front wheels, consequently greater
braking power is required at front
wheels in order to equalize the
braking effect at front and rear
wheels. Greater force is applied
to front brake shoes by using
larger wheel cylinders, so that
distribution of braking power is
approximately 56% at front wheels
and 44% at rear wheels.
When the brake pedal is released,
the master cylinder spring forces
the pedal back until the push rod
contacts the stop plate in master
cylinder. This spring also forces
the piston and primary cup to
follow the push rod and presses
the check valve firmly against its
seat.
At start of a fast release the
piston moves faster than fluid can
follow it in returning from the
pipes and wheel cylinders, there-
fore, a partial vacuum is momen-
tarily created in the pressure
chamber. Fluid supplied through
the breather port is then drawn
through the bleeder holes in
piston head and past the primary
cup to keep the pressure chamber
filled. See Figure 9-9, View C.
As pressure drops in master cyl-
inder, the shoe springs retract all
brake shoes and the connecting
links push the wheel cylinder
pistons inward, forcing fluid back
to master cylinder. Pressure of
returning fluid causes a rubber
disc to close all holes in the
check valve and forces the check
valve off its seat against the ten-
sion of master cylinder spring;
fluid then flows around the check
valve into the pressure chamber.
With the piston bearing against
the stop plate and the lip of the
primary cup just clear of the
compensating port, excess fluid
which entered through the bleeder
holes, or was created by expan-
sion due to increased tempera-
ture, now returns to reservoir
through the uncovered compensat-
ing port. See Figure 9-9, View D.
When pressure in wheel cylinders
and pipes becomes slightly less
than the tension of master cylin-
der spring, the check valve re-
turns to its seat on head nut to
hold 8 to 16 pounds per square
inch of "static" pressure in the
pipes and cylinders.