On double-actuated brakes, the anchor (solid) plungers should be positioned at the trailing end of each shoe (where they
will absorb the brake torque during forward wheel rotation). This will position the adjustable plungers at the leading end
of the shoes. Also note that the shoe web is unsymetrical. The long-radius end should be engaged with the adjustable
The air chamber should be screwed into the wedge bore of the plunger housing to a depth such that the wedge is ready
to lift the plungers off of the abutment seats at the first movement of the diaphragm (or piston). This provides the least
lost motion and maximum useful chamber (or piston) stroke.
This unit is designed to "bottom-out" in the wedge bore and provide this optimum adjustment automatically.
Whenever the power unit is removed from the brake, the wedge assembly may also become dislodged. Before
reinstalling the power unit, reposition the wedge assembly so that the rollers and roller cage are engaged in the plunger
slots. This is accomplished automatically by simply aligning the two ears on wedge spring retainer with corresponding
grooves in the wedge bore of the plunger housing.
All Fail-Safe units are equipped with manual caging bolts to permit safe handling for service work. These bolts are
sealed and prevented from rotating due to vibration.
When the brakes are equipped with Fail-Safe units, cage the power springs before starting any
disassembly or removal of wheels and drums. After parts are all reassembled and in place,
uncage the power springs before returning the vehicle to service.
When a vehicle is disabled due to low or lost air pressure, block wheels and cage the power springs before moving the
Manually Caging and Uncaging Fail-Safe Units: On the standard Fail-Safe, first loosen the boot clamp screw and
remove the rubber boot (if one is used). Then loosen the caging bolt-lock screws and swing the lock out of the way.
The power spring is manually caged (compressed) by turning the caging bolt clockwise approximately 18 to 21 full turns.
Do not force the bolt beyond its normal stop. Uncaging (releasing) the spring is accomplished by reversing the procedure
again do not force the bolt beyond its normal stop. If desired, both the manual caging and uncaging operation can be
made easier by applying air pressure to the Fail-Safe chamber to take the spring load off the caging bolt. Fail-Safe Units
can also be caged by air using the vehicle emergency air system or other available air supply. Apply and hold 70 psi air
pressure through the parking port of the Fail-Safe Unit to hold the power spring compressed.
On standard Fail-Safe units, after manually uncaging the spring, swing boltlock back and secure in place. Clean cap and
boot and install boot with vent slot at bottom.