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MAIN SPRING
DIAPHRAGM
NON-PRESSURE
HOUSING
FIGURE 3 - PIGGYBACK SPRING BRAKE CUT-AWAY VIEW
5. Following the vehicle manufacturer's recom-
mended procedures, deactivate the electrical sys-
tem in a manner that safely removes all electrical
power from the vehicle.
6. Never exceed manufacturer's recommended
pressures.
7. Never connect or disconnect a hose or line con-
taining pressure; it may whip. Never remove a com-
ponent or plug unless you are certain all system
pressure has been depleted.
8. Use only genuine Bendix
ponents and kits. Replacement hardware, tubing,
hose, fittings, etc. must be of equivalent size, type
and strength as original equipment and be
designed specifically for such applications and
systems.
9. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
not attempt repairs requiring machining or weld-
ing unless specifically stated and approved by the
vehicle and component manufacturer.
10. Prior to returning the vehicle to service, make
certain all components and systems are restored
to their proper operating condition.
EMERGENCY
PORT
RELEASE
TOOL
SIDE
POCKET
PUSH ROD
®
replacement parts, com-
OPERATION: GENERAL
The Bendix Piggyback Spring Brake is made up of the service
chamber and the piggyback chamber. The Bendix Piggyback
Spring Brake provides service braking, parking, and
emergency braking.
The spring brake can be mounted with the mounting bolts in
either a vertical or horizontal plane on standard mounting
stud centers. Two air lines are used: the service air line and
the emergency air line.
Controlled air pressure enters the service chamber through
the inlet port and acts upon the diaphragm moving the push
plate and rod assembly forward.
When the service chamber is used to actuate cam type
brake foundation assemblies, the yoke (which is threaded
on the push rod) is connected to a slack adjuster, which in
turn is connected to the brake cam shaft. This forward motion
of the push rod rotates the slack adjuster, cam shaft and
cam applying the vehicle brakes.
The greater air pressure admitted to the service chamber,
the greater the force applied by the push rod and, conversely,
the less pressure applied to the service chamber the less
force applied by the push rod. Push rod force is determined
by multiplying the delivered air pressure by the effective
diaphragm area. For example, if 60 p.s.i. is admitted to a
type 30 service chamber, the lineal force on the end of the
push rod is approximately 1,800 lbs.
When air pressure is released from the brake chamber, the
push rod return spring in combination with the brake shoe
return spring returns the diaphragm, push plate and rod
assembly, slack adjuster and brake cam to their released
positions releasing the brakes.
When the driver operates the parking brake, air is exhausted
from the piggyback chamber. The main spring is allowed to
extend, which forces the push rod and the emergency
diaphragm forward. This forces the service diaphragm and
service push rod forward which applies the brakes.
When the air pressure in the piggyback chamber drops to
below 78 p.s.i., the main spring overcomes chamber
pressure and forces the push rod and emergency diaphragm
forward.
WARNING!
•
Piggyback Spring Brake Chambers contain a
loaded compression spring. Property damage,
serious injury or death may occur if instructions
are not followed completely.
•
When replacing the piggyback spring brake, make
sure to block the wheels to prevent vehicle
rollaway.
3
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