Pontiac FIREBIRD 1972 Service Manual page 467

TANDEM DIAPHRAGM POWER BRAKE-BENDIX 5C-7
With vacuum on the left side and atmospheric pressure on
the right side of both diaphragms, a force is developed to
move the vacuum power diaphragm and plate assembly,
hydraulic push rod and hydraulic piston to the left. This
movement closes the compensating port in the master
cylinder and forces brake fluid under pressure through the
brake tubes into the brake wheel cylinders.
As hydraulic pressure is developed in the master cylinder,
a counter-force (to the right), acting through the push rod,
sets up a reaction force against the hub of the diaphragm
plates and against the valve plunger through a rubber
reaction disc at the end of the hydraulic push rod piston.
The rubber disc reacts like hydraulic fluid to distribute the
pressure between the diaphragm plate hub and the valve
plunger head in proportion to their respective contact
areas. This reaction pressure acting against the valve
plunger and valve operating rod tends to move the valve
plunger slightly to the right, in relation to the valve hous-
ing, to close the atmospheric port.
Since part of the counter-force (to the right) reacts
through the valve plunger and valve operating rod against
the driver's foot, a feel of the braking effort is provided.
The amount of reaction transmitted to the valve plunger
is designed into the power brake to assure maximum
power consistent with the assurance that the driver always
maintains a feel of the amount of brake that is being
applied. This reaction force is in direct proportion to the
hydraulic pressure developed within the brake system.
HOLDING POSITION (Fig. 5C-5)
During application of the brakes, the reaction against the
valve plunger is working against the driver, attempting to
close the atmospheric port. When both the vacuum and
atmospheric ports are closed, the power brake is in the
holding position and any degree of braking application
attained will be held until pedal pressure is increased or
decreased. This holding position occurs whenever brake
pedal pressure is held constant for a moment; however, as
described above, when the power section is fully applied,
the valve plunger is held away from the atmospheric port
seat to admit full atmospheric pressure, as shown in Fig.
32-4. When the brake pedal is released, the diaphragm
and plate assembly and the hydraulic pistons are returned
by their return springs to the released position and fluid
from the wheel cylinders and brake tubes flows slowly
back to the master cylinder sections and reservoirs.
NOTE: BOTH HYDRA ULIC PISTONS
MUST RETURN FAR ENOUGH TO OPEN
BOTH COMPENSA TING PORTS, BUT
THE HYDRA ULIC PUSH ROD SHOULD
AL WA YS REMAIN IN CONTACT WITH
THE REAR PISTON.
Both fluid reservoirs, cast integrally with the master cylin-
der, supply fluid to the space around its piston between the
primary and secondary seals through the fluid inlet
(bypass) port in the casting. When the brake pedal is
released quickly, the two return springs in the master
cylinder force the hydraulic pistons to the released posi-
tion faster than fluid can flow from the wheel cylinders.
Fluid compensation is provided by a flow of fluid from the
space between the primary cup and secondary seal
through holes in the head of the piston and around the
edge of each primary cup. The excess fluid then in the
system flows back to the reservoirs through the compen-
sating ports. The secondary seal on the rear piston pre-
vents loss of fluid from the hub end of the master cylinder
bore. The secondary seal on the front piston that faces
toward the rear system prevents any flow of fluid from the
rear system to the front. The other secondary seal on the
front piston, facing toward the front, prevents any fluid
flow from the front system to the rear.
The front or floating piston supplies the correct fluid dis-
placement for the front wheel brake requirements. The
rear piston supplies the correct fluid displacement for the
rear wheel brake requirements. The hydraulic pressure
developed in both systems is equal at all times since the
front or floating piston is balanced between the hydraulic
pressure in both systems. If the front system fails, the front
piston and spring bottom against the end of the bore and
then the rear piston develops hydraulic pressure to the
rear wheels. If the rear system fails, the rear piston and
spring bottom against the front piston and then mechani-
cally force the front piston forward to develop hydraulic
pressure to the front wheels. Failure in either system has
no effect on the other system but is immediately evident
to the driver because of the additional pedal travel re-
quired to actuate the remaining half of the dual brake
system. (In addition, a brake warning light switch also
actuates the parking brake light on the instrument panel.)
LOSS OF VACUUM POWER
In the event of engine failure, the vacuum chambers
within the power brake provide adequate vacuum reserve
for two or three brake applications. If the vacuum check
valve is defective or after braking has depleted the vacuum
reserve, the driver can still operate the brakes manually
but pedal effort is considerably greater.
CHECKS AND ADJUSTMENTS ON CAR
1. Check for free operation of brake pedal. If binding
exists, check all pivot points for binding and lubricate
,
as required.
2.' Check stop light switch for proper setting and operat-
ion. See Section 5.
3. Check fluid level in hydraulic cylinder reservoirs. See
Fig. 5-1 1.