PAGE 17
lever as far as it will go and hold it in place with the
locking device.
Dual Parking Control Valves. When main air
pressure is lost, the spring brakes come on. Some
vehicles, such as buses, have a separate air tank
which can be used to release the spring brakes.
This is so you can move the vehicle in an
emergency. One of the valves is a push-pull type
and is used to put on the spring brakes for parking.
The other valve is spring loaded in the "out" position.
When you push the control in, air from the separate
air tank releases the spring brakes so you can
move. When you release the button, the spring
brakes come on again. There is only enough air in
the separate tank to do this a few times. Therefore,
plan carefully when moving. Otherwise, you may be
stopped in a dangerous location when the separate
air supply runs out. See Figure 5.3.
Figure 5.3
5.1.16 – Antilock Braking Systems (ABS)
Truck tractors with air brakes built on or after March
1, 1997, and other air brakes vehicles, (trucks,
buses, trailers, and converter dollies) built on or
after March 1, 1998, are required to be equipped
with antilock brakes. Many commercial vehicles
built before these dates have been voluntarily
equipped with ABS. Check the certification label for
the date of manufacture to determine if your vehicle
is equipped with ABS. ABS is a computerized
system that keeps your wheels from locking up
during hard brake applications.
Vehicles with ABS have yellow malfunction lamps to
tell you if something isn’t working.
Tractors, trucks, and buses will have yellow ABS
malfunction lamps on the instrument panel.
Trailers will have yellow ABS malfunction lamps on
the left side, either on the front or rear corner.
Dollies manufactured on or after March 1, 1998 are
required to have a lamp on the left side.
On newer vehicles, the malfunction lamp comes on
at start-up for a bulb check, and then goes out
quickly. On older systems, the lamp could stay on
until you are driving over five mph.
If the lamp stays on after the bulb check, or goes on
once you are under way, you may have lost ABS
control at one or more wheels.
In the case of towed units manufactured before it
was required by the Department of Transportation,
it may be difficult to tell if the unit is equipped with
ABS. Look under the vehicle for the electronic
control unit (ECU) and wheel speed sensor wires
coming from the back of the brakes.
ABS is an addition to your normal brakes. It does
not decrease or increase your normal braking
capability. ABS only activates when wheels are
about to lock up.
ABS does not necessarily shorten your stopping
distance, but it does help you keep the vehicle under
control during hard braking
Section 5 – Air Brakes Page 5-4
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Figure 5.4
.Subsection 5.1
Test Your Knowledge
1. Why must air tanks be drained?
2. What is a supply pressure gauge used for?
3. All vehicles with air brakes must have a low
air pressure warning signal. True or False?
4. What are spring brakes?
5. Front wheel brakes are good under all
conditions. True or False?
6. How do you know if your vehicle is
equipped with antilock brakes?
These questions may be on your test. If you can’t
answer them all, re-read subsection 5.1.
5.2 – Dual Air Brake
Most heavy-duty vehicles use dual air brake
systems for safety. A dual air brake system has two
separate air brake systems, which use a single set
of brake controls. Each system has its own air tanks,
hoses, lines, etc. One system typically operates the
regular brakes on the rear axle or axles. The other
system operates the regular brakes on the front axle
(and possibly one rear axle). Both systems supply
air to the trailer (if there is one). The first system is
called the "primary" system. The other is called the
"secondary" system. See Figure 5.4.
Before driving a vehicle with a dual air system, allow
time for the air compressor to build up a minimum of
100 psi pressure in both the primary and secondary
systems. Watch the primary and secondary air
pressure gauges (or needles, if the system has two
needles in one gauge). Pay attention to the low air
pressure warning light and buzzer. The warning
light and buzzer should shut off when air pressure
in both systems rises to a value set by the
manufacturer. This value must be greater than 55
psi.
The warning light and buzzer should come on
before the air pressure drops below 55 psi in either
system. If this happens while driving, you should
stop right away and safely park the vehicle. If one
air system is very low on pressure, either the front
or the rear brakes will not be operating fully. This
means it will take you longer to stop. Bring the
vehicle to a safe stop, and have the air brakes
system fixed.
Section 5 – Air Brakes Page 5-5
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
5.3 – Inspecting Air Brake Systems
You should use the basic seven-step inspection
procedure described in Section 2 to inspect your
vehicle. There are more things to inspect on a
vehicle with air brakes than one without them.
These things are discussed below, in the order they
fit into the seven-step method.
5.3.1 – During Step 2 Engine Compartment
Checks
Check Air Compressor Drive Belt (if compressor is
belt-driven). If the air compressor is belt-driven,
check the condition and tightness of the belt. It
should be in good condition.
5.3.2 – During Step 5 Walk-around Inspection
Check Slack Adjusters on S-cam Brakes. Park on
level ground and chock the wheels to prevent the
vehicle from moving. Release the parking brakes so
you can move the slack adjusters. Use gloves and
pull hard on each slack adjuster that you can reach.
If a slack adjuster moves more than about one inch
where the push rod attaches to it, it probably needs
adjustment. Adjust it or have it adjusted. Vehicles
with too much brake slack can be very hard to stop.
Out-of-adjustment brakes are the most common
problem found in roadside inspections. Be safe.
Check the slack adjusters.
All vehicles built since 1994 have automatic slack
adjustors. Even though automatic slack adjustors
adjust themselves during full brake applications,
they must be checked.
Automatic adjusters should not have to be manually
adjusted except when performing maintenance on
the brakes and during installation of the slack
adjusters. In a vehicle equipped with automatic
adjusters, when the pushrod stroke exceeds the
legal brake adjustment limit, it is an indication that a
mechanical problem exists in the adjuster itself, a
problem with the related foundation brake
components, or that the adjuster was improperly
installed.
The manual adjustment of an automatic adjuster to
bring a brake pushrod stroke within legal limits is
generally masking a mechanical problem and is not
fixing it. Further, routine adjustment of most
automatic adjusters will likely result in premature
wear of the adjuster itself. It is recommended that
when brakes equipped with automatic adjusters are
found to be out of adjustment, the driver take the
vehicle to a repair facility as soon as possible to
have the problem corrected. The manual
adjustment of automatic slack adjusters is
dangerous because it may give the driver a false
sense of security regarding the effectiveness of the
braking system.
The manual adjustment of an automatic adjuster
should only be used as a temporary measure to
correct the adjustment in an emergency situation as
it is likely the brake will soon be back out of
adjustment since this procedure usually does not fix
the underlying adjustment problem.
(Note: Automatic slack adjusters are made by
different manufacturers and do not all operate the
same. Therefore, the specific manufacturer’s
Service Manual should be consulted prior to
troubleshooting a brake adjustment problem.)
Check Brake Drums (or Discs), Linings, and
Hoses. Brake drums (or discs) must not have
cracks longer than one half the width of the friction
area. Linings (friction material) must not be loose or
soaked with oil or grease. They must not be
dangerously thin. Mechanical parts must be in
place, not broken or missing. Check the air hoses
connected to the brake chambers to make sure they
aren't cut or worn due to rubbing.
5.3.3 – Step 7 Final Air Brake Check
Do the following checks instead of the hydraulic
brake check shown in Section 2, Step 7: Check
Brake System.
Check Air Compressor Governor Cut-in and
Cut-out Pressures. Pumping by the air
compressor should start at about 100 psi and stop
at about 125 psi. (Check manufacturer's
specifications.) Run the engine at a fast idle. The
air governor should cut-out the air compressor at
about the manufacturer's specified pressure. The air
pressure shown by your gauge(s) will stop rising.
With the engine idling, step on and off the brake to
reduce the air tank pressure. The compressor
should cut-in at about the manufacturer's specified
cut-in pressure. The pressure should begin to rise.
If the air governor does not work as described
above, it may need to be fixed. A governor that does
not work properly may not keep enough air pressure
for safe driving.
Test Air Leakage Rate. With a fully-charged air
system (typically 125 psi), turn off the engine,
release the parking brake (push in); and time the air
pressure drop. The loss rate should be less than
two psi in one minute for single vehicles and less
than three psi in one minute for combination
vehicles.
With the air pressure built up to governor cutoff (120
– 140 psi), shut off the engine, chock your wheels (if
necessary), release the parking brake (all vehicles),
Section 5 – Air Brakes Page 5-6
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
and the tractor protection valve (combination
vehicle); and fully apply the foot brake. Hold the foot
brake for one minute. Check the air gauge to see if
the air pressure drops more than three pounds in
one minute (single vehicle) or four pounds in one
minute (combination vehicle). If the air pressure
falls more than three psi in one minute for single
vehicles (more than four psi for combination
vehicles), the air loss rate is too much. Check for air
leaks and fix before driving the vehicle. Otherwise,
you could lose your brakes while driving.
Test Low Pressure Warning Signal. Shut the
engine off when you have enough air pressure so
that the low pressure warning signal is not on. Turn
the electrical power on and step on and off the brake
pedal to reduce air tank pressure. The low air
pressure warning signal must come on before the
pressure drops to less than 55 psi in the air tank (or
tank with the lowest air pressure, in dual air
systems). See Figure 5.5.
If the warning signal doesn't work, you could lose air
pressure and you would not know it. This could
cause sudden emergency braking in a single-circuit
air system. In dual systems the stopping distance
will be increased. Only limited braking can be done
before the spring brakes come on.
Figure 5.5
Check That Spring Brakes Come On
Automatically. Continue to fan off the air pressure
by stepping on and off the brake pedal to reduce
tank pressure. The tractor protection valve and
parking brake valve should close (pop out) on a
tractor-trailer combination vehicle and the parking
brake valve should close (pop out) on other
combination and single vehicle types when the air
pressure falls to the manufacturer’s specification
(20 – 45 psi). This will cause the spring brakes to
come on.
Check Rate of Air Pressure Buildup. When the
engine is at operating rpms, the pressure should
build from 85 to 100 psi within 45 seconds in dual
air systems. (If the vehicle has larger than minimum
air tanks, the buildup time can be longer and still be
safe. Check the manufacturer's specifications.) In
single air systems (pre-1975), typical requirements
are pressure build-up from 50 to 90 psi within 3
minutes with the engine at an idle speed of 600-900
rpms.
If air pressure does not build up fast enough, your
pressure may drop too low during driving, requiring
an emergency stop. Don't drive until you get the
problem fixed.
Test Parking Brake. Stop the vehicle, put the
parking brake on, and gently pull against it in a low
gear to test that the parking brake will hold.
Test Service Brakes. Wait for normal air pressure,
release the parking brake, move the vehicle forward
slowly (about five mph), and apply the brakes firmly
using the brake pedal. Note any vehicle "pulling" to
one side, unusual feel, or delayed stopping action.
This test may show you problems, which you
otherwise wouldn't know about until you needed the
brakes on the road.
Subsections 5.2 and 5.3
Test Your Knowledge
1. What is a dual air brake system?
2. What are the slack adjusters?
3. How can you check slack adjusters?
4. How can you test the low pressure warning
signal?
5. How can you check that the spring brakes come
on automatically?
6. What are the maximum leakage rates?
These questions may be on your test. If you can’t
answer them all, re-read subsections 5.2 and 5.3.
5.4 – Using Air Brakes
5.4.1 – Normal Stops
Push the brake pedal down. Control the pressure so
the vehicle comes to a smooth, safe stop. If you
Section 5 – Air Brakes Page 5-7
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
have a manual transmission, don't push the clutch
in until the engine rpm is down close to idle. When
stopped, select a starting gear.
5.4.2 – Braking with Antilock Brakes
When you brake hard on slippery surfaces in a
vehicle without ABS, your wheels may lock up.
When your steering wheels lock up, you lose
steering control. When your other wheels lock up,
you may skid, jackknife, or even spin the vehicle.
ABS helps you avoid wheel lock up. The computer
senses impending lockup, reduces the braking
pressure to a safe level, and you maintain control.
You may or may not be able to stop faster with ABS,
but you should be able to steer around an obstacle
while braking, and avoid skids caused by over
braking.
Having ABS on only the tractor, only the trailer, or
even on only one axle, still gives you more control
over the vehicle during braking. Brake normally.
When only the tractor has ABS, you should be able
to maintain steering control, and there is less
chance of jackknifing. But, keep your eye on the
trailer and let up on the brakes (if you can safely do
so) if it begins to swing out.
When only the trailer has ABS, the trailer is less
likely to swing out, but if you lose steering control or
start a tractor jackknife, let up on the brakes (if you
can safely do so) until you gain control.
When you drive a tractor-trailer combination with
ABS, you should brake as you always have. In other
words:
Use only the braking force necessary to stop safely
and stay in control.
Brake the same way, regardless of whether you
have ABS on the tractor, the trailer, or both.
As you slow down, monitor your tractor and trailer
and back off the brakes (if it is safe to do so) to
stay in control.
There is only one exception to this procedure, if you
always drive a straight truck or combination with
working ABS on all axles, in an emergency stop, you
can fully apply the brakes.
Without ABS, you still have normal brake functions.
Drive and brake as you always have.
Remember, if your ABS malfunctions, you still have
regular brakes. Drive normally, but get the system
serviced soon.
5.4.3 – Emergency Stops
If somebody suddenly pulls out in front of you, your
natural response is to hit the brakes. This is a good
response if there's enough distance to stop, and you
use the brakes correctly.
You should brake in a way that will keep your
vehicle in a straight line and allow you to turn if it
becomes necessary. You can use the "controlled
braking" method or the "stab braking" method.
Controlled Braking. With this method, you apply
the brakes as hard as you can without locking the
wheels. Keep steering wheel movements very small
while doing this. If you need to make a larger
steering adjustment or if the wheels lock, release
the brakes. Re-apply the brakes as soon as you
can.
Stab Braking. Apply your brakes all the way.
Release brakes when wheels lock up. As soon as
the wheels start rolling, apply the brakes fully again.
(It can take up to one second for the wheels to start
rolling after you release the brakes. If you re-apply
the brakes before the wheels start rolling, the
vehicle won't straighten out.)
5.4.4 – Stopping Distance
Stopping distance was described in Section 2 under
"Speed and Stopping Distance." With air brakes
there is an added delay - “Brake Lag”. This is the
time required for the brakes to work after the brake
pedal is pushed. With hydraulic brakes (used on
cars and light/medium trucks), the brakes work
instantly. However, with air brakes, it takes a little
time (one half second or more) for the air to flow
through the lines to the brakes. Thus, the total
stopping distance for vehicles with air brake
systems is made up of four different factors.
Perception Distance + Reaction Distance + Brake
Lag Distance + Braking Distance = Total Stopping
Distance.
The air brake lag distance at 55 mph on dry
pavement adds about 32 feet. So at 55 mph for an
average driver under good traction and brake
conditions, the total stopping distance is over 450
feet. See Figure 5.6.
Section 5 – Air Brakes Page 5-8
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
PAGE 17