chap75 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 75, the reader should
be able to:

•
•
•

Prepare for ASE Brakes (A5) certification test content area
“B” (Drum Brake Diagnosis and Repair).
Discuss the procedure recommended for brake drum

removal.
Discuss the inspection and lubrication points of the backing
plate.

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OBJECTIVES:
After studying Chapter 75, the reader should
be able to:

•
•

Explain the importance of the proper drum brake
hardware.
Disassemble and reassemble a drum brake assembly.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
bearingized • brake hardware kits
core
speed nuts
tinnerman nuts

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BRAKE DRUM DIAGNOSTIC PROCEDURE
Diagnosing brake concern includes the following steps:
Step #1 Verify the customer complaint (concern). For example,
common drum brake concerns include:
•
•
•
•

Low brake pedal

Parking brake does not hold the vehicle on a hill
Noise while braking
Noise when the vehicle is moving, but brakes are not applied

Step #2 Perform a visual inspection of the brakes and related
parts, such as the wheels, tires, and suspension system.

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Step #3 Determine the root cause. For example, this could include
determining that the parking brake is corroded causing the brake
shoes to wear. Replacing just the brake shoes will not correct the

root cause.
Step #4 Restore the brake system to like-new operation, which is
the purpose of any repair.
Step #5 Test drive the vehicle to verify that the service has
corrected the customer’s complaint.

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Drum brake service usually involves the following steps:
Step #1 Removing the brake drums.
Step #2 Inspect the brake drums for damage and measure the
inside diameter and compare to specifications.

Step #3 Inspect brake linings and hardware for wear or damage.
Step #4 Inspect the wheel cylinder and brake lines for leakage.
Step #5 Check the backing plate for excessive rust or wear.

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BRAKE DRUM REMOVAL
The drum has to be removed before inspection or repair of a drum
brake can begin. There are two types of drums, and removal
procedure depends on which type is being serviced.
With either type it is recommended that the drums be marked with
an “L” for left or an “R” for right so that they can be replaced in

the same location.

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CAUTION: Proper precaution should be taken to prevent any asbestos
that may be present in the brake system from becoming airborne. Removal
of the brake drum should occur inside a sealed vacuum enclosure equipped
with a HEPA filter or washed with water or solvent.

Figure 75–1
A liquid soaking solvent, such as brake cleaner,
should be used to wet the linings. The purpose

of wetting the lining material while the drum is
still on the vehicle is to prevent the possibility
of asbestos from the lining becoming airborne.
Asbestos is only hazardous when asbestos
dust is airborne and is breathed in during
brake system service.

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Hub or Fixed Drums A fixed or hub-mounted drum is often used
on the rear of front-wheel-drive vehicles. The drum has a hub for
inner and outer bearings and is retained by a spindle nut.
To remove the brake drum, remove the dust cap and cotter key that

is used to retain the spindle nut. Remove spindle nut and washer
and the brake drum can be carefully pulled off the spindle.
CAUTION: Overheating or not allowing the drum to cool slowly can
cause the brake drum to distort. Using a puller can also damage a drum.

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Hubless or Floating Drums Floating or hubless drums are
usually used on the rear of a rear-wheel-drive vehicle. Drums are
secured to the axle flange by the wheel and lug nuts. New vehicles
have tinnerman nuts (clips), called speed nuts, on the stud.
These thin sheet-metal nuts keep the brake drum from falling off

during shipping and handling prior to installation of the rear
wheels.
See Figure 75–2.

NOTE: Use of an air hammer with a flat-headed driver against the hub
also works well to break the drum loose from the hub.

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Figure 75–2 Tinnerman nuts are used at the vehicle assembly plant to prevent the brake drum
falling off until the wheels can be installed. These sheet-metal retainers can be discarded after
removal.

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The tinnerman nuts can be discarded because they are not needed
after the vehicle leaves the assembly plant. After removing the
wheels, the drum should move freely on the hub and slip off over
the brake shoes. Some drum brakes have two threaded holes in the
drum that allow bolts to be installed.

Figure 75–3 Turning the bolts that
are threaded into the brake drum
forces the drum off the hub.

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By tightening the bolts, the drum is forced off of the hub. Two
situations can prevent the drum from being removed:
1. The Drum Is Rusted to the Hub The fit between drum and
hub is very close because it is this center pilot hole in the
drum that centers the drum on the axle. Rust and corrosion
often causes the drum to seize to the hub. Striking the area
inside the wheel studs will usually break the drum loose from
the hub. Sometimes a torch has to be used to expand the pilot
hole.

2. The Brake Shoes Are Worn into the Drum Even if the pilot
hole is loose, many brake drums cannot be removed because
the inner edge of the brake drum catches on the lining. Pulling
outward often bends the backing plate or breaks some of the
mounting hardware. To prevent damage, back off the adjuster.
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Figure 75–4 If the brake shoes have worn into the brake drum, the adjuster can be backed in
after removing the access plug. After removing the plug, use another wire or screwdriver to
move the adjusting lever away from the starwheel, then turn the starwheel with a brake
adjusting tool, often called a brake “spoon.” (Courtesy of Allied Signal Automotive Aftermarket)

NOTE: Be sure to reinstall the adjuster opening plugs. These plugs help
keep water and debris out of the brakes.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DRUM BRAKE DIASSEMBLY
After removing brake drums, they should be cleaned, inspected,
measured, and possibly machined before return to service.
Brake shoes and other brake hardware should be wetted down with
a solvent or enclosed in an approved evacuation system to prevent
possible asbestos release into the air.

NOTE: There are generally no “exact” disassembly or reassembly
procedures specified by the manufacturer. The order in which the parts are

disassembled or reinstalled is based on experience and the personal
preference of the tech.

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Cutting the Nails Trick
Many times a brake drum cannot be removed because the linings have worn a
groove into the drum. Attempting to adjust the brakes inward is often a frustrating
and time-consuming operation. The easy solution is to use a pair of diagonal
sidecut pliers and cut the heads off the hold-down pins (nails) at the backing plate.
This releases the brake shoes from the backing plate and allows enough
movement of the shoes to permit removal of the brake drum without

bending the backing plate. The hold-down pins (nails) must obviously be
replaced, but they are included in most
drum brake hardware kits. Since most
brake experts recommend replacing all
drum brake hardware anyway, this
solution does not cost any more than
normal, may save the backing plate
from damage, and saves time.
Figure 75–5 Using side-cut pliers to cut the heads off the
hold-down spring pins (nails) from the backing plate to
release the drum from the shoes.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Usually, the first step in the disassembly of a drum brake system is
removal of the return (retracting) springs.

Figure 75–6 Using a brake spring tool to release a
return (retracting) spring from the anchor pin.
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After the return springs have been removed, the hold-down springs
and other brake hardware can be removed.
Figure 75–7 A special tool,
called a hold-down spring
tool, being used to depress

and rotate the retainer.

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INSPECTING THE BACKING PLATE
The backing plate supports the parts of the drum brake and helps to
keep water from getting onto the brake shoes.
The backing plate bolts to the rear axle or spindle and is made from
stamped steel. Backing plates are plated (usually cadmium) or
painted to prevent rusting.
When brakes are serviced, the six raised contact surfaces, called
pads, ledges, or shoe contact areas, of the backing plate should be

inspected because they rub against the sides of the shoes.
See Figure 75–8.

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Figure 75–8 A typical rusting backing plate shoe pad. This can cause the brakes to squeak when
the shoes move outward during a brake application and again when the brake pedal is released.

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Figure 75–9 The backing plate is the foundation of every drum brake.

If the pads are worn more
than 1/16 in. (1.5 mm),
the backing plate should
be replaced. The backing
plate must be inspected
for looseness or bending.

The raised pads should be
cleaned and lubricated.

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Figure 75–10 Lithium high-temperature brake grease is used by many service technicians to
lubricate metal-to-metal contact surfaces of a drum brake. A small metal-handled acid brush is
stuck through a hole cut into the lid of this container of grease to make the application of
grease easy and less messy. Avoid using too much grease. Excessive grease can get onto the
friction surfaces of the brake shoes or drum and affect braking performance.

Lithium high-temperature
brake grease, synthetic brake
grease, silicone grease, or
antiseize should be used to
lubricate drum brake parts.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DRUM BRAKE LINING INSPECTION
Both primary (front facing) and secondary (rear facing) lining
material must be thicker than 0.060 in. (1.5 mm).
NOTE: Most vehicle and brake lining manufacturers recommend
replacing worn brake lining when the thickness of the riveted lining
reaches 0.060 in. or less. An American nickel is about 0.060 in. thick, so
remember you must always have at least “a nickel’s worth of lining.”

Figure 75–11 A rule of thumb is that the lining should be at least the thickness of a nickel.
Continued
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If a riveted brake lining is cracked between rivets, the lining
should be replaced. Some brake manufacturers recommend
riveted replacement linings because of reduced brake noise.
These manufacturers say that the recessed rivet holes allow brake
dust to pocket away from the lining surface and thus help prevent
the linings from glazing.
Most experts agree that high-quality linings purchased from a
known brake manufacturer will help assure that the brake
performance is returned to original equipment standards.
The lining must be replaced if cracked, as shown in Figure 75–12.

Continued

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