TRUCK SERVICE MANUAL
These self-adjusting type brakes have two single end
wheel cylinders, each operating a forward acting shoe
providing directional braking response. The closed end of the
wheel cylinder has a slotted ramp which is the self-centering
sliding anchor for the opposite shoe. The two wheel cylinder
pistons apply equal amounts of hydraulic force to each shoe
Shoes are adjustable manually by means of two hex head
friction spring locked studs, exposed on the backing plate
(Fig. 1). Each stud rotates an adjuster cam located under a
brake shoe table. Shoe hold-downs consist of two plain
washers, multi-rattle wave washer between and a C-clip fitted
over the extended shank of the cam stud. Two retracting
springs are hooked between the shoes and have their longer
shanks connected to the anchor end of the shoes.
The function of the contact plug type self adjusting
mechanism is to maintain a nearly constant previously set
running clearance between brake shoe and drum throughout
the life of the brake lining.
The self adjusting mechanism is contained entirely in the
brake shoe. This mechanism incorporates a contact plug
located in the center of the lining and is the lining wear
sensing component. This plug is pinned (lever actuating pin)
in a free rotatable manner to the adjusting lever, which in turn
makes contact with the manually operated adjusting cam.
This lever in turn is pinned (lever pivot pin) in a rotatable
manner through a hole in the shoe web. As lining wear
progresses, the contact plug is gradually depressed upon
successive brake applications and a spring-actuated wedge
with serrated teeth on the angular surface advance between
the lever-actuating pin and a stationary but rotatable serrated
wedge pin, thereby maintaining the adjustment.
Force is exerted by the lever actuating spring on the lever
actuating pin acting through the wedge and wedge guide pin
so as to maintain firm contact between these parts while the
brake is in the released position.
Shoe adjustment is accomplished by the adjusting lever
changing position with relation to the shoe web as the contact
plug is depressed. The adjusting lever then comes to rest
against the adjusting cam, causing the shoe to remain in the
advanced or adjusted position.
BRAKE SHOE REMOVAL
Position vehicle on floor stands so that the wheels
are free to turn.
Remove wheel hub and drum assemblies.
Secure pistons in wheel cylinders with cylinder
Remove brake shoe retracting springs.
Remove brake shoe guide "C" washer, two shoe
guide washers and wave washer from each shoe.
Refer to Fig. 3 (items 16, 17 and 18).
Pull shoes free from adjuster cams and then rotate
shoe from brake group freeing ends of wheel
After shoes are removed disassemble self adjuster
mechanism on each shoe:
Disconnect wedge (coil) spring from finger on
wedge and remove spring.
Unhook torsion adjuster spring from shoe
Work torsion adjuster spring coil from lever
pivot pin and slide U-hook off plug-lever pin.
Pull adjuster lever from opposite side of shoe
and contact plug out of shoe table, freeing
wedge retaining washer, wedge and wedge
CLEANING AND INSPECTION
Thoroughly clean all parts (except drums) free of dirt and
grease with cleaning solvent and wipe dry.
Because studies have indicated that exposure to
excessive amounts of asbestos dust may be a potential health
hazard, OSHA has set maximum limits of levels of airborne
asbestos dust to which workers may be exposed. Since most
automotive friction materials normally contain a sizable
amount of asbestos, it is important that people who handle
brake linings be aware of the problem and know the
precautions to be taken.
OSHA standards should be consulted with respect to
mandatory requirements as well as for suggested procedures
to minimize exposure.
DRUMS, SHOES AND LINING
For complete details pertaining to reconditioning drums
and shoes, refer to CTS-2779.
(Cont'd. on next page)
CTS-2491T Page 3
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