chap77 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 77, the reader should
be able to:

•
•
•
•
•

Prepare for ASE Brakes (A5) certification test

content area “C” (Disc Brake Diagnosis and
Repair).
List the items that should be checked during a
visual inspection.
Describe the caliper disassembly procedure.
Describe the caliper assembly procedure.
List the steps necessary to reduce brake noise.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
abutments • antiseize compound

bedded-in • brake assembly fluid • burnished
constrained layer shims (CLS)
lithium-based brake grease • loaded calipers
minimum thickness • MOS2 (molybdenum disulfide)
grease

Continued
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KEY TERMS:
polyalphaolefin (PAO) • phenolic caliper pistons
reaction pads
silicone grease • synthetic grease

ways

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

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DISC BRAKE DIAGNOSTIC PROCEDURE
When diagnosing disc brake concerns, the first step is to verify 
the customer complaint. This usually includes test driving the 
vehicle to see if the complaint can be duplicated. If the problem 
cannot be duplicated, then the repair or service cannot be verified.
CAUTION: Do not test­drive the vehicle on public roads if the red 
brake warning light is on.

After verifying the customer concern, the brake system should be 

carefully inspected and all parts and systems checked for proper 
operation. 

Continued
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Step #1  Check the brake pedal height and verify proper operation. 
If the brake pedal is low, check the parking brake and count the 
number of “clicks.” There should be 3 to 7 clicks. If there are over 
10 clicks, check the rear brakes.
Step #2  Safely hoist the vehicle and remove the wheels. Visually 
check the following:
• Flexible brake lines for wear or damage

• Disc brake rotors for excessive rust or scoring
• Disc brake calipers for leakage or damage

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Step #3  Remove disc brake calipers and check the disc brakepads 
for proper lining thickness and check for cracks or other damage.
Step #4  Replace all components that do not meet factory specs.
Step #5  Test­drive the vehicle to verify that the repairs did correct 
the customer concern.
Let The Owner Drive
When verifying the customer complaint, ask the owner or driver of the

vehicle to drive. Often, the problem is best discovered if the vehicle is
being driven exactly the same way as when the complaint first occurred.
For example, the tech may brake harder or softer than the driver so the
problem may not be detected.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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VISUAL INSPECTION
Even with operating wear­indicating sensors, a visual inspection 
is very important. 

Figure 77–1
Minimum thickness for various

types of disc brake pads. Disc
brake pads can, of course, be
replaced before they wear down
to the factory-recommended
minimum thickness.

Continued
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A lining thickness check alone should not be the only inspection 
performed on a disc brake. A thorough visual inspection can only 
be accomplished by removing the friction pads.
Figure 77–2 This cracked disc brake pad must be replaced even though it is thicker than the

minimum allowed by the vehicle manufacturer.

Continued
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NOTE: Some disc brake pads use a heat barrier (thermo) layer between 
the steel backing plate and the friction material. The purpose of the heat 
barrier is to prevent heat from transferring into the caliper piston where it
may cause the brake fluid to boil. Do not confuse the thickness of the 
barrier as part of the thickness of the friction lining material. The barrier
material is usually a different color and usually can be distinguished from
the lining material.

Some disc brake pads may show more wear on the end of the pad 
that first contacts the rotor as compared to the pad trailing end.
This wear is caused by force between the pad and the abutment 
(slide area). In designs that place the caliper piston exactly in the 
center of the leading edge of the pad that first contacts the rotor as 
it is revolving through the caliper, pressures are often one­third 
higher than the average pressure exerted on the entire pad. The 
result of this is greater wear. 
Continued
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Figure 77–3 Be careful to observe the direction in which replacement linings are facing. Some
vehicle manufacturers offset the friction material on the steel backing to help prevent or

minimize tapered pad wear.

Continued
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Figure 77–4 Most disc brake calipers have a brake inspection opening. For a thorough
inspection, however, the caliper should be removed and the entire braking system thoroughly
inspected.

One method used to help reduce 
tapered pad wear is the design
that offsets the friction material
off center. 

Be certain to position the pads 
correctly or severe tapered pad 
wear will occur. 

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

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The Bleed and squirt Test
If you suspect a brake is not being fully released, simply loosen the bleeder
valve. If brake fluid squirts out under pressure, then the brake is being kept
applied. Look for a defective flexible brake hose. If the vehicle is off the
ground, the wheels should be able to be rotated with the brakes off. If a
wheel is difficult or hard to turn by hand and is easy to turn after opening

the bleeder valve, then there is a brake fluid restriction between the master
cylinder and the brake.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DISC BRAKE CALIPER SERVICE
Removal  Hoist the vehicle and remove the wheel(s). Note the 
caliper mount position as shown here before removing the caliper.
Figure 77–5 Both rear- and forward-mounted calipers have the bleeder valve at the top. Some
calipers will fit on the wrong side of the vehicle, yet not be able to be bled correctly because
the bleeder valve would point down, allowing trapped air to remain inside the caliper bore. If
both calipers are being removed at the same time, mark them “left” and “right.”

Knowing whether the caliper
is “rear mount” position or 
“forward mount” position is 
often needed when purchasing 
replacement calipers. 
Continued
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Figure 77–6 Many manufacturers recommend
removing one-half of the brake fluid from the
master cylinder before servicing disc brakes.
Use a squeeze bulb and dispose of the used
brake fluid correctly.

Remove the caliper following 
the steps described in
Figures 77–6 through 77–10.

Continued
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Figure 77–7 Most manufacturers recommend that the bleeder valve be opened and the brake
fluid forced into a container rather than back into the master cylinder reservoir. This helps
prevent contaminated brake fluid from being forced into the master cylinder where the dirt and
contamination could cause problems.

Continued
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Figure 77–8 Many calipers use a hollow “banjo bolt” to retain the flexible brake line to the
caliper housing. The fitting is usually round like a banjo. The copper washers should always be
replaced and not reused.

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Figure 77–9 Caliper retaining bolts are often called guide pins. These guide pins are used to
retain the caliper to the steering knuckle. These pins also slide through metal bushings and
rubber O-rings. (Courtesy of EIS Brake Parts)

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Figure 77–10 If the caliper is not being removed, it must be supported properly so that the
weight of the caliper is not pulling on the flexible rubber brake line. A suitable piece of wire,
such as a coat hanger, may be used.

Continued
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Inspection and Disassembly  Check for brake fluid in and around 
the piston boot area. If the boot is damaged or a fluid leak is 
visible, then a caliper assembly repair or replacement is required. 
Figure 77–11 A wooden block or a folded shop cloth helps prevent damage when caliper
pistons are removed.

Continued
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Figure 77–12 After the piston is removed from the caliper housing, the dust boot can often be
removed using a straight-blade screwdriver.

Continued
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Phenolic Caliper Pistons  Made from a phenol­formaldehyde resin 
combined with various reinforcing fibers.
When phenolic caliper pistons were first used, results were not 
good and the problem was blamed on “those darn plastic pistons.” 
The pistons were becoming stuck in the caliper, which caused the 
brake pads to remain applied. This caused the brake pads to wear out 
very rapidly.
The problem occurred because the phenolic pistons absorbed 
moisture and swelled in size.

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By reducing the diameter of the 
pistons and improving the 
caliper boot seal, the sticking 
problem has been solved. 

Figure 77–13 Phenolic (plastic) pistons
should be carefully inspected.

Since the mid­80s, phenolic 

caliper pistons have been used 
as original equipment by many 
vehicle manufacturers. 
They are natural insulators and 
keep heat from transferring to 
the brake fluid. 
Phenolic brake caliper pistons 
are lighter in weight than steel 
caliper pistons and are usually 
brown in color. 
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 77–14 (a) The outside surface of caliper pistons should be carefully inspected. The
square-cut O-ring inside the caliper rides on this outside surface of the piston. Sometimes dirty
pistons can be cleaned and reused. (b) If there are any surface flaws such as rust pits on the
piston, it should be replaced.

Steel Caliper Pistons
Many manufacturers still use 
steel pistons. 

The stamped steel pistons are 
plated first with nickel, then 
chrome to achieve the desired 
surface finish. 
Continued
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Unlike phenolic caliper pistons, steel pistons can transfer heat from 
the brake pads to the brake fluid. The surface finish on a steel 
piston is critical. Steel can rust and corrode. Any surface pitting 
can cause the piston to stick.
NOTE: Care should be taken when cleaning steel pistons. Use crocus

cloth to remove any surface staining. Do not use sandpaper, emery cloth,
or any other substance that may damage the chrome surface finish.

Continued
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