chap78 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 78, the reader should
be able to:
•

Prepare for ASE Brakes (A5) certification test content area “E”
(Miscellaneous Systems Diagnosis and Repair).

•
•

Describe what is required of a parking brake.
Describe the parts and operation of the parking brake as used on a rear
drum brake system.

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 78, the reader should
be able to:
•

Describe how a parking brake functions when the vehicle is equipped

with rear disc brakes.

•

Explain how to adjust a parking brake properly.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
application cables
control cables
electric parking brake (EPB) • equalizer
intermediate lever

red brake warning lamp
vacuum servo

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PARKING BRAKE
Before 1967, most vehicles had only a single master cylinder
operating all four brakes. If fluid leaked at just one wheel, the
operation of all brakes was lost. This required a separate method
to stop the vehicle in case of an emergency.
After 1967, federal regulations required use of dual or tandem
master cylinders where half of the braking system has its own
separate hydraulic system.

In case one-half of the system fails, a dash brake warning lamp
lets the driver know that a failure has occurred. The term parking
brake has replaced the term emergency brake since the change to
dual master cylinder design.
Continued
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According to Federal Motor Vehicle Safety Standard (FMVSS)
135, the parking brake must hold a fully loaded (laden) vehicle
stationary on a slope of 20% up or down grade.
The hand force required cannot exceed 80 lb. (18 N) or a foot force
greater than 100 lb. (22 N).
See Figure 78–1 for a typical parking brake system.

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Figure 78–1 Typical parking brake cable system showing the foot-operated parking brake lever
and cable routing.

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PEDALS, LEVERS AND HANDLES
Parking brakes are applied by
a pedal, a lever, or a handle
from inside the vehicle.

Foot pedals and floor-mounted
levers are the common means
of applying parking brakes.
See Figure 78–3.

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Figure 78–3 Typical hand-operated parking brake. Note that the adjustment for the cable is
underneath the vehicle at the equalizer.

All parking brakes are applied manually and the release procedure
varies with the design of the parking brake control.
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All parking brake controls incorporate a ratchet mechanism to lock
the brake in the applied position.

Figure 78–4 A ratchet mechanism is used to lock parking brakes in the applied position.

Some vehicles were equipped with a system that required the
driver to depress the parking brake pedal to release the parking
brake once it was set. The rubber pad on the parking brake pedal
usually states “push to release.”
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Parking Brake Pedals A parking brake pedal is applied by
depressing it with a foot. The ratchet engages automatically and
the pedal remains in the depressed position.
The pedal is released by a pull or a small T-handle or lever under
the dash. This disengages the ratchet mechanism, and allows a
return spring to move the pedal to the unapplied position.
On some vehicles, the release lever is integrated into the
underside of the dash and connects to the release mechanism
through a rod or cable.
See Figure 78–5.

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Figure 78–5 A remote-mounted parking brake release lever.

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Automatic Parking Brake Release Some vehicles with pedal-operated parking brakes have
an automatic release mechanism that disengages the parking brake using a vacuum servo
controlled by an electrical solenoid.

Figure 78–6 Automatic parking
brake release mechanisms
usually use a vacuum servo to
operate the release lever.

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Pump To Release
A customer called and asked a dealer for help because the parking brake could not
be released. The service technician discovered that the customer was attempting to
release the parking brake by depressing the parking brake pedal, as was done on
the customer’s previous vehicle. The service technician pulled on the release lever
and the parking brake was released.

A metal rod connects the vacuum servo to the upper end of the
parking brake release lever.
When the engine is running (to provide vacuum) and the shifter is
placed in gear, an electrical contact closes to energize the solenoid
and route vacuum to the servo.
The servo diaphragm then retracts the rod, which releases the
parking brake.
See Figure 78–7.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 78–7 The two plastic vacuum tubes on the steering column are used to release the
parking brake when the gear selector is moved from park into a drive gear.

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PARKING BRAKE WARNING LAMP
Whenever the parking brake is engaged, a red brake warning
lamp lights on the dash. On most vehicles, this is the same lamp

that lights when there is a hydraulic or brake fluid level problem.
The lamp for the parking brake warns the driver that the parking
brake is applied or partially applied. The warning helps prevent
damage or overheating to the brake drums and linings that could
occur if the vehicle was driven with the parking brake applied.
If the red BRAKE warning lamp is on, check the parking brake
to see if it is fully released. If the BRAKE lamp is still on, the
parking brake switch may be defective, out of adjustment, or
there may be a hydraulic problem.
Continued
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PARKING BRAKE LINKAGES

Parking brake linkages transmit force from the pedal, lever, or
handle inside the vehicle to the brake friction assemblies.
Linkage Rods Parking brake linkage rods made from solid steel
are commonly used with floor-mounted actuating levers to span
the short distance to an intermediate lever or an equalizer.
Linkage Cables Parking brake cable is made of woven-steel wire
encased in a reinforced rubber or plastic housing. The housing is
fixed in position at both ends, and is routed under the vehicle
through mounting brackets that hold the cable in position, yet allow
a small amount of movement. The cable slides back and forth inside
the housing to transmit application force.
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The ends of parking brake cables are fitted with a wide variety of
connectors that attach to actuating devices, other linkage parts, or
the wheel friction assemblies.
Control cables attach to the parking brake pedal, lever, or handle
inside the vehicle, and transmit force to an intermediate lever or
equalizer to the application cables. The application cables use the
force passed through the linkage to apply the friction assemblies.
See Figure 78–8.

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Figure 78–8 The cable from the activating lever to the equalizer is commonly called the control
cable. From the equalizer, the individual brake cables are often called application cables. These
individual cables can usually be purchased separately.

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Look for Swollen Parking Brake Cables
Always inspect parking brake cables for proper operation. A cable that is larger in
diameter in one section indicates that it is rusting inside and has swollen. A rusting
parking brake cable can keep the rear brake applied even though the parking brake
lever has been released. This can cause dragging brakes, reduced fuel economy,
and possible vehicle damage due to overheated brakes.

Figure 78–9 Notice how rust
inside the covering of this parking
brake cable has caused the cable
to swell.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Linkage Levers The rods and cables transmit application force
in direct proportion. If 50 lbs of application force is delivered to
one end of a rod or cable, 50 lbs of force will be available at the
other end as well.
Unfortunately, the amount of physical force a driver can apply to

the parking brake control is insufficient for effective parking
brake operation. For this reason, all parking brake linkages
contain one or more levers that increase application force.
See Figure 78–10.

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Parking brake pedals, floormounted levers, & pivoting
under dash handles are types
of levers to increase parking
brake application force.

Figure 78–10 Intermediate levers in the parking
brake linkage increase the application force.

Straight-pull parking brake
handles are not levers; they
are commonly connected to
other levers in the linkage.
A lever in the parking brake
linkage under the vehicle is
called an intermediate lever,
and provides leverage in
addition to that supplied by
the parking brake control.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Linkage Equalizers In some parking brake linkages, the rods or
cables to the two friction assemblies are adjusted separately.
If the adjustments are unequal, one brake will apply before the
other, preventing full lining-to-drum contact at the opposite wheel
and greatly reducing the holding power of the parking brake.
To prevent unequal application, most parking brake linkages use
an equalizer to balance the force from the parking brake control,
and transmit an equal amount to each friction assembly.
See Figure 78–11.

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Figure 78–11
A cable guide is a common type of parking brake linkage equalizer.

Equalizers come in many
shapes and sizes, but the
simplest is the cable guide
attached to a threaded rod.
This type of equalizer pivots
or allows the inner cable to
slide back and forth to even
out the application force.

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Another type of equalizer installs in a long application cable that
runs from the linkage at the front of the vehicle to one rear brake.

Figure 78–12 Some parking
brake equalizers are installed
in the brake cable.

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