chap74 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 74, the reader should
be able to:

•
•
•
•
•

Prepare for ASE Brakes (A5) certification test

content area “B” (Drum Brake Diagnosis and
Repair).
Identify drum brake component parts.
Describe the operation of non-servo brakes.
Explain the operation of dual-servo brakes.
Discuss drum brake adjusters.

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

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KEY TERMS:
anchor eyes • automatic adjusters
beehive holddown • bonded linings • brake fade • brake
shoe holddown • brake shoe return springs • brake shoes

core charge
double-trailing brake • dual-servo brake • duo-servo
gas fade
labyrinth seal • leading shoe • leading-trailing brakes •
ledges • lining edge codes • lining fade • lining table
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
mechanical fade • nibs • non-servo brakes • over-travel
spring
pawl • piston stops • platform • primary shoe
riveted linings

secondary shoe • self-energizing action • servo brake •
shoe anchors • shoe contact areas • shoe rim • shoe
support pads • shoe web
trailing shoe
water fade
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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DRUM BRAKE ADVANTAGES
Drum brakes were the first type of brakes used on motor vehicles. 
Today, over 100 years after the first “horseless carriages,” drum 
brakes are still used on the rear of most vehicles.

Figure 74–1 Typical brake

system components
showing disc brakes on the
front and drum brakes on
the rear.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 74–2 An exploded view of a typical drum brake assembly.

The drum brake has been more 
widely used than any other 
automotive brake design. 

Continued
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The drum brake has advantages that contribute to its widespread 
use on the rear axle of most automobiles.
Self­Energizing and Servo Action  Drum brakes can apply more 
stopping power for a given amount of force applied to the brake 
pedal disc brakes. 
This is possible because the drum brake design offers a self­
energizing action that helps force the brake linings tightly against 
the drum.
Some drum brake designs use an effect called servo action that 

enables one brake shoe to help apply the other for increased 
stopping power. 
Continued
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Parking Brake Service  Drum brakes make excellent parking 
brakes. A simple linkage fitted to the brake assembly allows 
relatively low effort from the driver to hold a heavy vehicle in 
place when parked.
Disc brakes, which do not benefit from self­energizing or servo 
action, require a complex set of extra parts to provide enough 
application force to work well as parking brakes.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DRUM BRAKE DISADVANTAGES
Drum brake disadvantages fall into three areas:
Brake fade
Brake adjustment
Brake pull
Brake Fade  The greatest drawback of drum brakes is that they 
are susceptible to fade. Brake fade is loss of stopping power 
when excessive heat reduces the friction between shoe linings
and the brake drum.

Continued

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

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Mechanical Fade  When the brake drum gets so hot it expands 
away from the brake linings mechanical fade occurs. The shoes 
move outward to maintain contact with the drum, causing the 
brake pedal to drop toward the floor as additional brake fluid 
moves into the hydraulic system.
Lining Fade  When the friction coefficient of the brake lining 
material drops off sharply, lining fade occurs, because intense 
heat makes it “slippery.” The pedal becomes hard and there is a 
noticeable loss of braking power.
Gas Fade  Rare type gas fade occurs under extended hard braking 
from high speeds. A thin layer of hot gasses and dust particles can 

build up between the brake shoe linings and drum, acts as a 
lubricant and reduces friction. The brake pedal is hard.
Continued
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Water Fade  A drum brake friction assembly cannot be made 
waterproof because clearance is necessary between the rotating 
drum and the fixed backing plate.
This clearance allows a small amount of air circulation that helps 
combat heat fade, but it can also allow water to enter the friction 
assembly. Water fade occurs when moisture is trapped between 
the shoes and drum, where it acts as a lubricant. 

Quick-and-Easy Drum Brake Adjustment
Check
Tap the brake drum lightly with a hammer or wrench. If the brake shoes
are not contacting the drum, the drum will ring like a bell. If the shoes are
contacting the drum, the sound will be muffled.
Continued
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Brake Adjustment  As the brake shoe lining material wears, the 
clearance between the linings and drum increases, resulting in 
longer brake pedal travel. To maintain a high brake pedal, a 
mechanism must be included in the friction assembly for periodic 
adjustment of the clearance between the shoe and drum. 

Brake Pull  The final disadvantage of drum brakes is that they 
sometimes pull the vehicle to one side or the other during braking. 
Brake pull occurs when the friction assemblies on opposite sides 
of the vehicle have different amounts of stopping power. These 
differences can be caused by brake fade or misadjustment of the 
clearance between the brake linings and drum.
Continued
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DRUM BRAKE PARTS
A drum brake assembly consists 
of backing plate and springs, 
brake shoes and adjuster.

The foundation of every drum 
brake is the backing plate.
It mounts to the steering 
knuckle on the front brakes;
to the suspension or axle 
housing on the rear brakes. 
Figure 74–3 The backing plate is the
foundation of every drum brake.
Continued
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The backing plate serves as the mounting surface for all the other 

friction assembly parts. The backing plate also functions as a dust 
and water shield to keep contaminants out of the brake assembly.
The edge of the backing plate curves outward to form a lip that 
strengthens the backing plate and fits inside the brake drum to help 
prevent water entry.
The lip fits into a machined groove in the open edge of the brake 
drum to provide an even better water barrier or seal.
This is called a labyrinth seal. 
See Figure 74–4.

Continued
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Continued

Figure 74–4 The flange on the backing plate is
designed to come close to a notch or groove on the
brake drum, forming a type of seal that helps prevent
debris and water from getting onto the drum brake.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Shoe Anchors  The shoes are prevented from rotating with the 
drum when the brakes are applied by shoe anchors  Most drum 
brakes have a single anchor; some use two or more.
Many anchors are a simple round post that is permanently mounted 

on the backing plate. The brake shoes have semicircular cutouts 
where they contact the anchor, and the anchor positively locates the 
shoe on the backing plate.
Another type of anchor is the self­centering or keystone anchor. It is 
called a keystone anchor because of the angled shape similar to a 
keystone used on the top center of a stone arch.
See Figure 74–5.

Continued
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Figure 74–5 A keystone anchor allows the brake shoes to self-center in the drum.

Continued
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Piston Stops  Some backing plates incorporate piston stops that 
prevent the wheel cylinder pistons from coming out of their bores 
when the friction assembly is disassembled for servicing.
The stops may be part of a reinforcing plate positioned under the 
anchor or they can be stamped directly into the shape of the 
backing plate itself.
When piston stops are used, the wheel cylinder must be removed 
from the backing plate before it can be taken apart for servicing.
See Figure 74–6.

Continued
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Figure 74–6 Piston stops prevent the wheel cylinder from coming apart

Continued
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Shoe Support Pads  The shoe support pads are stamped into 
the backing plate and contact the edges of the brake shoes to 
keep the linings properly aligned with the center of the friction 
surface inside the brake drum.
These pads are also called ledges or shoe contact areas.
The support pads are slightly coated with special high 
temperature silicone brake grease to minimize wear, prevent rust, 
and eliminate squeaking that can occur when the shoes move 
slightly on the pads during a stop.

Continued
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Wheel Cylinders  
Hydraulic pressure
is transferred from
the master cylinder
to each wheel cylinder 
through brake fluid.

Force exerted on the 
brake fluid by the 
driver forces the 
piston inside the 
wheel cylinder to 
move outward. 
Figure 74–7 Cross-section of a wheel cylinder that shows all of its internal parts. The brake line
attaches to the fluid inlet. The cup expander prevents the cup seal lip from collapsing when the
brakes are released.
Continued
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Figure 74–8
The pushrods are held
in place by the rubber
dust boots.
As the wheel cylinder
pistons move outward,
the pushrods transfer
the movement to the
brake shoes.

Through pushrods or links, this movement acts on the brake shoes, 
forcing them outward against the brake drum. 

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DRUM BRAKE SHOES
Linings of drum 
brakes are attached 
to curved metal 
assemblies called 
brake shoes. 
Figure 74–9 Steel brake
shoes are made from two
stampings welded together.

Most shoes are made 

of two pieces of 
sheet steel welded 
together in a T­
shaped cross­section. 
Continued
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While aluminum shoes are lighter than their steel counterparts, 
they are more expensive to make and not as durable at high 
temperatures.
The outer edge is lined with a friction material that contacts the 
brake drum to generate the actual stopping power.
The ends of the linings on most brake shoes are tapered to prevent 

vibration and brake noise as shown in Figure 74–10.

Continued
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Figure 74–10 Tapered ends on the brake linings reduce noise.

The curved metal piece 
on the outer portion of 
the shoe is called the 
lining table, the shoe 
rim or platform. 

On some, the lining table
edge has small V­ or U­
shaped notches called nibs. 
Continued
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