Air Brake Adjustment, Chapter 8, Driving Commercial Vehicles Tri Port Adjustment
User Manual: TriPort
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203
What You’ll Learn
After reading this chapter you will be able to:
explain why air brake adjustment needs to be regularly
checked
explain how to measure air brake chamber push rod travel
and how to adjust air brakes
Brake Adjustment — It’s Critical
The most important part of any braking system is the foundation brakes.
If these are not in correct working order and properly adjusted, the best
designed air brake system won’t be able to safely stop the vehicle.
Unfortunately, brake adjustment is often neglected. The reality is that
brake failures and runaways that result in crashes are almost never
caused by an air system failure, but by the absence of routine brake
maintenance, or by the driver failing to check brakes on a daily basis.
Daily pre- and post-trip inspections — including checking and adjusting
brakes — are mandatory. The driver may be held responsible if the brakes
are incorrectly adjusted or not working properly.
Brake adjustment — it’s the law
Federal and provincial laws require that manual and automatic slack
adjusters be checked daily, during the pre-trip inspection. It is up to you,
the professional driver, to ensure your vehicle has safe, correctly adjusted
brakes.
You’re also required by law to check your brakes before driving down
steep grades that are posted with regulatory signs.
You can’t always crawl underneath your vehicle to measure air brake
chamber pushrod travel, but there are devices available to help you
visually check for brake adjustment.
Air Brake Adjustment
8
Your company may have
a maintenance crew to
keep vehicles safely
running.
But one person alone is
ultimately responsible to
ensure that the brakes
are operating properly
before the vehicle is
moved. That person is
the driver.
Fast Fact
You may be fi ned as
much as $2,000 if you
are operating a vehicle
with brakes that are
defective. This includes
excessive brake chamber
pushrod stroke.
Fast Fact
204204
A typical S-cam brake. The brake is
shown in the released position.
S-cam brake — released
The most common type of air brake chamber used on trucks and trailers
is the Type 30. These chambers have a maximum available stroke of
21/2 inches. A long stroke Type 30 is designated Type 30LS, and has a
maximum available stroke of three inches.
S-cam brake — applied
When correctly adjusted,
the running clearance
between the brake lining
and drum is only a few
thousandths of an inch
— hardly thicker than
three sheets of paper.
Fast Fact
A typical S-cam brake. The brake is
shown in the applied position.
Chapter 8
Slack adjustment means
adjusting the brakes
to keep air chamber
pushrod travel within
tolerance to ensure that
the brakes are effective.
Pushrod travel means
the distance the pushrod
extends from the air
brake chamber when the
brakes are applied.
Defi nition
In this diagram, the brakes have been applied and the brake linings have
contacted the brake drum. The brake chamber has stroked less than half
of its maximum stroke, indicating that the brake is in correct adjustment.
205205
S-cam brake — incorrectly adjusted and
cold brake drum
In this diagram, the brakes are applied and the linings have contacted the
brake drum.
RoadSense Tip
Check air brake chamber
pushrod travel at least
daily.
Each brake on an axle
should be adjusted to
have a similar amount of
pushrod travel — that
is, the pushrod travel of
the left brake should be
similar to the pushrod
travel of the right brake
on the axle.
Air Brake Adjustment
Once you learn how,
manual slack adjusters
are easy to adjust.
Air brake equipped
vehicles rarely “lose”
their brakes. Rather,
brake failure occurs
because the driver has
failed to check and
correctly adjust the
brakes.
Fast Fact
This brake chamber pushrod has
excessive travel, but may work as
long as the brake drum is cool. But
it won’t stay cool for long!
Note that the brake chamber has stroked more than half of its maximum,
which means that this air brake chamber pushrod has excessive travel
and the brakes are in need of adjustment.
When the brake drum is cool and with normal brake application
pressure, the brake will seem to be effective, so it is easy to be lulled into
a false sense of security.
206
S-cam brake — incorrectly adjusted and
hot brake drum
This diagram shows the same brake after a few brake applications.
Cast iron brake drums expand when heated, causing the air chamber to
stroke further and further as the temperature rises.
If an unforeseen event required the driver to make a sudden stop, the
brake chamber could bottom out, and braking power would be greatly
reduced.
On long downgrades, the expansion of hot brake drums can cause a total
loss of braking and result in a runaway.
Now that the brake drum is hot,
the brake linings will no longer
contact the drum. The brake
chamber has bottomed out.
Warning
Even incorrectly adjusted
brakes may seem to work
when brakes are lightly
applied.
This could result in a
dangerously performing
brake system under
moderate to heavy
braking, or on a
downhill.
Chapter 8
207
Checking and Adjusting S-Cam Brakes —
Manual Slack Adjuster
Drivers seldom have the luxury of having the use of a pit and hydraulic
jacks so that brake adjustment can be done with wheels off the ground.
However, brakes can still be adjusted very accurately with the wheels on
the ground using one of the following methods.
Be sure that the vehicle is safely parked with the wheels blocked. The air
system should be at full pressure and the spring parking brakes released.
Turn the engine off so that you can listen for air leaks.
There are two methods of checking for correct adjustment, but the
measurements that indicate the need for adjustment are different.
Pry method of free stroke measurement
This picture shows the common method of checking the free stroke of the
pushrod:
Air Brake Adjustment
Using a brake tool to measure air
brake chamber pushrod free stroke.
To begin, make sure system air pressure is over 100 p.s.i. (690 kPa) and
all parking brakes are released.
1. Make a mark on the pushrod where it exits the brake chamber.
Note: It is sometimes diffi cult to mark the pushrod at this
location. Alternatively, mark the centre of the clevis pin on the
slack adjuster arm.
2. Pull the pushrod out from the brake chamber, using a tool for
leverage.
3. Measure the distance between the chalk mark and the face of
the brake chamber – 1/2 to 3/4 of an inch of free stroke is a good
range. If this distance is more than 3/4 of an inch, the brakes
need adjustment.
RoadSense Tip
Make sure you have the
proper equipment to
check and adjust the
brakes. You will need:
• a fl ashlight to
inspect components
• chalk or other
marker, and a ruler
or other measuring
device to check
pushrod free stroke
• a tool to pry on the
air brake chamber
pushrod to check for
free stroke
• a wrench to adjust
air brakes
You should also wear
sturdy clothing, a hard
hat and eye protection.
RoadSense Tip
Always block the wheels
before checking any
brakes.
Turn the engine off so
that you can also listen
for any air leaks.
208
Applied stroke method (service brake application)
The applied stroke (brake application) method requires two people
— one to apply the brakes and one to measure travel.
Checking the pushrod stroke by
applying the brakes.
If the red marking on the pushrod
(at the chamber face) is visible,
it indicates that the brake is
dangerously out of adjustment and
it needs immediate attention.
If you have a device to apply and hold the service brakes on, you can use
the brake application method without needing another person.
Brake adjustment indicators
New air brake chamber pushrods have a marking (usually red) to indicate
when brake adjustment must be done immediately. If the pushrod travel
becomes excessive, the marking will show.
Don’t wait until the red marking is exposed before adjusting the brakes.
The applied stroke method is the method used by roadside inspectors,
and is also a method recommended by commercial fl eet maintenance
supervisors.
1. With the brakes released, make a mark where the pushrod exits
the brake chamber.
2. With the engine off, make a series of brake applications to
reduce the reservoir pressure to between 90 to 100 p.s.i. (620 to
690 kPa).
Chapter 8
209
3. Apply and hold a full brake application (90 to 100 p.s.i.).
4. Measure the distance between the mark and the face of the
brake chamber or the centre of the clevis pin. The difference
between measurements is called the chamber applied stroke.
Brake Adjustment — Manual Slack Adjuster
With a typical Type 30 clamp type air chamber, you must adjust the
brakes if pushrod travel is:
• more than
3/4 of an inch using the pry method of free stroke
measurement, or
• more than 13/4 of an inch using the brake application — applied
stroke measurement method.
Note that these are maximum measurements. You should adjust the
brakes if your measurements approach these limits.
RoadSense Tip
While checking air
brakes look for other
brake component wear
and excessive play
including:
• s-cam bushing wear
• s-cam spline wear
• slack adjuster play
(worn gear)
• clevis pin wear
Adjustment
Most manual slack adjusters have a spring-loaded locking sleeve that
must be pressed in and held so the adjusting bolt can be turned.
Depending upon the orientation of the slack adjuster, the correct
direction to turn the adjusting bolt may be clockwise or counter-
clockwise. There are two indicators to watch for to ensure that you are
tightening and not loosening the slack adjustment.
With a wrench of the proper size, usually a 9/16 inch, depress the locking
sleeve and turn the adjusting bolt while watching the end of the camshaft.
The camshaft will rotate slightly as the bolt is being turned. If you are
Air Brake Adjustment
RoadSense Tip
While an adjustable
wrench may work, it is
better to use a closed
end wrench to adjust a
slack adjuster.
210
turning in the right direction, the cam will rotate in the same direction
that it would if the brakes were being applied, as shown by the arrow on
the cam end in the diagram below.
If when turning the adjusting bolt, the slack adjuster is pulling the
pushrod out of the chamber, stop. The adjusting bolt is being turned in
the wrong direction.
The push rod and the slack adjuster arm should never move in or out
while turning the adjusting bolt.
If the push rod goes into the chamber, this indicates that the previous
slack adjustment was done incorrectly.
Once the proper direction is established, continue turning until solid
resistance is met. This indicates that the brake linings have contacted the
brake drum.
If the brake has no dust shields, or if you can see the brake shoes and
linings through an inspection slot, you can visually verify that the linings
have contacted the drum.
Backing off the adjusting bolt about 1/3 of a turn should establish correct
running clearance between the lining and drum. Be sure the locking
sleeve re-engages the bolt so that the adjustment will not back off.
It is common, especially on tandem axle units, for the adjusting bolts
on one axle to adjust in one direction, while the other axle requires an
opposite turn. It is not uncommon to fi nd that the brakes on one or the
other axle have been mistakenly backed off, creating a serious safety
hazard.
After adjusting, verify that there is suffi cient clearance by re-measuring
the free stroke.
If the pushrod goes into the
chamber this indicates that the
previous slack adjustment was
done incorrectly.
Chapter 8
RoadSense Tip
Have an adjusting
wrench at hand when
checking the brakes
so that you can make
a brake adjustment if
required.
211
S-Cam Brakes — Automatic Slack Adjuster
All commercial trucks and trailers with air brakes have been
manufactured with automatic slack adjusters since 1996. Automatic
slack adjusters adjust themselves during full brake applications and are
able to maintain brake stroke more reliably than manual slack adjusters.
However, automatic slack adjusters must still be checked as part of a pre-
trip inspection.
Once properly installed, automatic slack adjusters should not need
manual adjustment. If an automatic slack adjuster is found to stroke
beyond the maximum allowed, this usually indicates that there are
other brake problems that need to be repaired by qualifi ed brake service
personnel.
Manual adjustment of automatic slack adjusters is dangerous because
it gives a false sense of security about the effectiveness of the braking
system. While a manual adjustment may temporarily shorten the stroke,
the automatic slack will soon re-set to its designed stroke.
Repeated manual adjustment can cause undue wear on the internal
components of the slack adjuster and possibly lead to early failure.
Manufacturers generally recommend that automatic slack adjusters be
checked by a mechanic at every chassis lubrication interval, every 40,000
kilometres, or every three months, whichever comes fi rst.
The National Safety Code of Canada as well as American and Mexican
laws require a daily check of brake adjustment as part of a pre-trip
inspection. As well, in certain mountainous areas of North America,
signs are posted requiring trucks to stop and check brakes before
proceeding down long grades. These brake checks are required
regardless of whether manual or automatic slack adjusters are used.
If, during one of these checks, a pushrod stroke is excessive, the
automatic slack adjuster has either failed, been incorrectly installed, or
there is a problem within the foundation brake.
Automatic slack
adjusters still need to
be regularly checked
to ensure that correct
adjustment is being
maintained.
Fast Fact
Warning
Automatic slack adjusters
should only be adjusted
or repaired by a qualifi ed
mechanic.
Follow the manufacturer’s
instructions in carrying
out any adjustment.
It is dangerous to manually
adjust automatic slack
adjusters.
Repeated adjustment can
mask other problems and
may lead to brake failure.
Warning
Air Brake Adjustment
212
Warning
Manually adjusting
automatic slack
adjusters is dangerous
and should only be
done by qualifi ed brake
service personnel.
Four types of automatic slack adjusters.
Checking and Adjusting Automatic Slack
Adjusters
Most manufacturers of automatic slack adjusters specify that pushrod
stroke be checked by making a 90 to 100 p.s.i. (620 to 690 kPa)
application. If you have no application pressure gauge, turn the engine
off and pump the reservoir pressure down to between 90 and 100 p.s.i.
- then make a full application.
Pushrod strokes with automatic slack adjusters are usually slightly longer
than with well-adjusted manual slack adjusters. With a typical Type 30
clamp type air chamber and an automatic slack adjuster, the brakes need
repair if pushrod measurements are:
• more than
3/4 of an inch using the pry method of free stroke
measurement, or
• more than two inches using the brake application - applied
stroke method.
Chapter 8
RoadSense Tip
It is strongly
recommended that you
obtain the service data
books for the make
of automatic slack
adjusters that you are
using.
They contain all the
information needed to
keep them operating
properly, including
lubrication requirements,
initial setup, inspection
and testing. They are
available where truck
parts are sold.
If a pushrod stroke is excessive, the automatic slack adjuster has either
failed, been incorrectly installed, or there is a problem within the
foundation brake.
THE FOLLOWING ARE EMERGENCY PROCEDURES ONLY. A REPAIR OR REPLACEMENT
MUST BE MADE AS SOON AS POSSIBLE.
Be sure that the vehicle is safely parked with the wheels blocked. The air
system should be at full pressure and the spring parking brakes released.
If you make an emergency adjustment of an automatic slack adjuster, be
sure to record it on your daily post-trip inspection report. After adjusting,
verify that there is suffi cient clearance by re-measuring the free stroke.
Warning
It is dangerous to manually
adjust automatic slack
adjusters.
The adjustment procedures
shown here are for use
in emergencies as a last
resort where qualifi ed
brake personnel can not
attend.
Remember – you, the
driver, are responsible
for the condition of the
vehicle you are operating
- including the brakes.
213
Slack adjusters with hexagonal adjusting bolts
If the slack adjuster has a hexagonal (six-sided) adjusting bolt, the brakes
may be set up by turning the adjusting bolt in a clockwise direction until
the lining contacts the drum. Backing off the adjusting bolt by 1/2 a turn
should restore running clearance. Backing off may take considerable
force and may be accompanied by a ratcheting sound and feel. This is
normal.
Slack adjusters with square adjusting bolts
If the slack adjuster has a square adjusting bolt located at the bottom
end of the body, do not attempt adjusting until a spring-loaded pawl
that meshes with internal teeth is disengaged. These units have a 3/4
inch hexagonal (hex) cap located on the slack adjuster body. Some of
these hex caps are equipped with a round “button” that can be pried up
approximately 1/32 of an inch and held, using a screwdriver. On units not
equipped with the button, the hex cap, spring and pawl must be removed.
With the spring and pawl disengaged, an adjustment can be made.
These brakes must be set up with a counter-clockwise turn until the
lining contacts the drum. Turning the adjusting bolt 1/
2 a turn clockwise
will restore running clearance. Release the button or re-install the spring
and pawl if they were removed.
These two types of automatic slack
adjusters both have hexagonal
adjusting bolts.
There is no reliable
substitute for physically
checking brake
adjustment.
Fast Fact
This type of slack adjuster has a
square adjusting bolt.
Air Brake Adjustment
214
Air Brake Adjustment Myths
There is some misinformation about air brakes that you may hear. Most of
this is not critical, but the following myths could be dangerous, if believed.
Myth #1: Brake adjustment can be checked from the cab by
making a full brake application and checking for
an initial pressure drop of between 8–12 p.s.i. (55.2
to 82.8 kPa). The assumption is that as the brake
chambers stroke further and further, that more air
volume will be required, and this should show up on
the reservoir gauges.
Fact #1: Modern trucks have very large air reservoirs, and
even if all the brakes had excess pushrod travel,
the pressure drop would not reach the 8–12 p.s.i.
range. Also, most truck reservoir gauges do not have
markings fi ne enough to accurately estimate such
pressure changes.
Myth #2: With the brakes applied, a 90-degree angle between
the centre of the slack adjuster arm and the chamber
pushrod is a good indication that the brake is in correct
adjustment.
Fact #2: The 90-degree angle is more dependent on the length
of the chamber pushrod than on brake adjustment.
Also, to prevent interference between the slack
adjuster and suspension parts, some manufacturers
will vary the angle up to plus or minus 10 degrees.
Myth #3: A clockwise turn of the adjusting bolt on a manual
slack adjuster will set up the brakes.
Fact #3: Depending on the orientation of the slack adjuster
on the brake assembly, the correct direction to set up
the brakes may be clockwise, or counter-clockwise.
On vehicles with tandem axles, it is common to fi nd
that one axle sets up with a clockwise turn, while the
other requires a reverse direction.
Myth #4: As long as the parking brake control valve is open and
the trailer supply valve is open (charged), spring brakes
are off and slack adjustment can be checked.
Fact #4: Some parking brake control valves and trailer supply
valves will remain in the open position with as little
as 20 p.s.i. (138 kPa) system air pressure. Yet the
spring brakes may be partially or fully applied. For
this reason, make sure system air pressure is at least
90–100 p.s.i. and all parking brakes are released
before checking brake adjustment.
Chapter 8
215
Review Questions
1. Why is it so critical to check slack adjustment?
2. Who is ultimately responsible for the brakes on a vehicle?
3. What is the fi ne for operating a commercial vehicle with
brakes improperly adjusted?
4. What are the dangers of operating a vehicle where the
pushrod travel is barely within tolerance when the brake
drums are cold?
5. What items should you carry to measure and adjust slack
adjusters?
6. What is the fi rst thing you should do when preparing to
measure slack adjustment?
7. How much pressure should you apply to the foot valve when
measuring slack adjustment using the applied stroke method?
8. How do you know you are turning the adjusting bolt in the
correct direction when adjusting a slack adjuster?
9. What is an advantage of automatic slack adjusters?
Air Brake Adjustment
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