PAGE 19
If you do cross the air-lines, supply air will be sent
to the service line instead of going to charge the
trailer air tanks. Air will not be available to release
the trailer spring brakes (parking brakes). If the
spring brakes don't release when you push the
trailer air supply control, check the air-line
connections.
Older trailers do not have spring brakes. If the air
supply in the trailer air tank has leaked away there
will be no emergency brakes, and the trailer wheels
will turn freely. If you crossed the air-lines, you could
drive away but you wouldn't have trailer brakes. This
would be very dangerous. Always test the trailer
brakes before driving with the hand valve or by
pulling the air supply (tractor protection valve)
control. Pull gently against them in a low gear to
make sure the brakes work.
Some vehicles have "dead end" or dummy couplers
to which the hoses may be attached when they are
not in use. This will prevent water and dirt from
getting into the coupler and the air-lines. Use the
dummy couplers when the air-lines are not
connected to a trailer. If there are no dummy
couplers, the glad hands can sometimes be locked
together (depending on the couplings). It is very
important to keep the air supply clean.
6.2.6 – Trailer Air Tanks
Each trailer and converter dolly has one or more air
tanks. They are filled by the emergency (supply) line
from the tractor. They provide the air pressure used
to operate trailer brakes. Air pressure is sent from
the air tanks to the brakes by relay valves.
The pressure in the service line tells how much
pressure the relay valves should send to the trailer
brakes. The pressure in the service line is controlled
by the brake pedal (and the trailer hand brake).
It is important that you don't let water and oil build
up in the air tanks. If you do, the brakes may not
work correctly. Each tank has a drain valve on it and
you should drain each tank every day. If your tanks
have automatic drains, they will keep most moisture
out. But you should still open the drains to make
sure.
6.2.7 – Shut-off Valves
Shut-off valves (also called cut-out cocks) are used
in the service and supply air-lines at the back of
trailers used to tow other trailers. These valves
permit closing the air-lines off when another trailer
is not being towed. You must check that all shut-off
valves are in the open position except the ones at
the back of the last trailer, which must be closed.
6.2.8 – Trailer Service, Parking and Emergency
Brakes
Newer trailers have spring brakes just like trucks
and truck tractors. However, converter dollies and
trailers built before 1975 are not required to have
spring brakes. Those that do not have spring brakes
have emergency brakes, which work from the air
stored in the trailer air tank. The emergency brakes
come on whenever air pressure in the emergency
line is lost. These trailers have no parking brake.
The emergency brakes come on whenever the air
supply knob is pulled out or the trailer is
disconnected. A major leak in the emergency line
will cause the tractor protection valve to close and
the trailer emergency brakes to come on. But the
brakes will hold only as long as there is air pressure
in the trailer air tank. Eventually, the air will leak
away and then there will be no brakes. Therefore, it
is very important for safety that you use wheel
chocks when you park trailers without spring
brakes.
You may not notice a major leak in the service line
until you try to put the brakes on. Then, the air loss
from the leak will lower the air tank pressure quickly.
If it goes low enough, the trailer emergency brakes
will come on.
Subsection 6.2
Test Your Knowledge
1. Why should you not use the trailer hand
valve while driving?
2. Describe what the trailer air supply control
does.
3. Describe what the service line is for.
4. What is the emergency air-line for?
5. Why should you use chocks when parking
a trailer without spring brakes?
6. Where are shut-off valves?
These questions may be on your test. If you can’t
answer them all, re-read subsection 6.2.
Section 6 - Combination Vehicles Page 6-6
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
6.3 – Antilock Brake Systems
6.3.1 – Trailers Required to Have ABS
All trailers and converter dollies built on or after
March 1, 1998, are required to have ABS. However,
many trailers and converter dollies built before this
date have been voluntarily equipped with ABS.
Trailers will have yellow ABS malfunction lamps on
the left side, either on the front or rear corner. See
Figure 6.7. Dollies manufactured on or after March
1, 1998, are required to have a lamp on the left side.
In the case of vehicles manufactured before the
required date, it may be difficult to tell if the unit is
equipped with ABS. Look under the vehicle for the
ECU and wheel speed sensor wires coming from
the back of the brakes.
Figure 6.7
6.3.2 – Braking with ABS
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.
ABS helps you avoid wheel lock up. The computer
senses impending lockup, reduces the braking
pressure to a safe level, and you maintain control.
Having ABS on only the trailer, or even on only one
axle, still gives you more control over the vehicle
during braking.
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.
Remember, if your ABS malfunctions, you still have
regular brakes. Drive normally, but get the system
serviced soon.
ABS won’t allow you to drive faster, follow more
closely, or drive less carefully.
6.4 – Coupling and Uncoupling
Knowing how to couple and uncouple correctly is
basic to safe operation of combination vehicles.
Wrong coupling and uncoupling can be very
dangerous. General coupling and uncoupling steps
are listed below. There are differences between
different rigs, so learn the details of coupling and
uncoupling the truck(s) you will operate.
6.4.1 – Coupling Tractor-Semitrailers
Step 1. Inspect Fifth Wheel
Check for damaged/missing parts.
Check to see that mounting to tractor is secure, no
cracks in frame, etc.
Be sure that the fifth wheel plate is greased as
required. Failure to keep the fifth wheel plate
lubricated could cause steering problems because
of friction between the tractor and trailer.
Check if fifth wheel is in proper position for
coupling:
Wheel tilted down toward rear of tractor.
Section 6 - Combination Vehicles Page 6-7
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Jaws open.
Safety unlocking handle in the automatic lock
position.
If you have a sliding fifth wheel, make sure it is
locked.
Make sure the trailer kingpin is not bent or broken.
Step 2. Inspect Area and Chock Wheels
Make sure area around the vehicle is clear.
Be sure trailer wheels are chocked or spring
brakes are on.
Check that cargo (if any) is secured against
movement due to tractor being coupled to the
trailer.
Step 3. Position Tractor
Put the tractor directly in front of the trailer. (Never
back under the trailer at an angle because you
might push the trailer sideways and break the
landing gear.)
Check position, using outside mirrors, by looking
down both sides of the trailer.
Step 4. Back Slowly
Back until fifth wheel just touches the trailer.
Don't hit the trailer.
Step 5. Secure Tractor
Put on the parking brake.
Put transmission in neutral.
Step 6. Check Trailer Height
The trailer should be low enough that it is raised
slightly by the tractor when the tractor is backed
under it. Raise or lower the trailer as needed. (If
the trailer is too low, the tractor may strike and
damage the trailer nose; if the trailer is too high, it
may not couple correctly.)
Check that the kingpin and fifth wheel are aligned.
Step 7. Connect Air-lines to Trailer
Check glad hand seals and connect tractor
emergency air-line to trailer emergency glad hand.
Check glad hand seals and connect tractor service
air-line to trailer service glad hand.
Make sure air-lines are safely supported where
they won't be crushed or caught while tractor is
backing under the trailer.
Step 8. Supply Air to Trailer
From cab, push in "air supply" knob or move
tractor protection valve control from the
"emergency" to the "normal" position to supply air
to the trailer brake system.
Wait until the air pressure is normal.
Check brake system for crossed air-lines.
Shut engine off so you can hear the brakes.
Apply and release trailer brakes and listen for sound
of trailer brakes being applied and released. You
should hear the brakes move when applied and air
escape when the brakes are released.
Check air brake system pressure gauge for signs of
major air loss.
When you are sure trailer brakes are working, start
engine.
Make sure air pressure is up to normal.
Step 9. Lock Trailer Brakes
Pull out the "air supply" knob or move the tractor
protection valve control from "normal" to
"emergency."
Step 10. Back Under Trailer
Use lowest reverse gear.
Back tractor slowly under trailer to avoid hitting the
kingpin too hard.
Stop when the kingpin is locked into the fifth wheel.
Step 11. Check Connection for Security
Raise trailer landing gear slightly off ground.
Pull tractor gently forward while the trailer brakes
are still locked to check that the trailer is locked
onto the tractor.
Step 12. Secure Vehicle
Put transmission in neutral.
Put parking brakes on.
Shut off engine and take key with you so someone
else won't move truck while you are under it.
Step 13. Inspect Coupling
Use a flashlight, if necessary.
Make sure there is no space between upper and
lower fifth wheel. If there is space, something is
wrong (kingpin may be on top of the closed fifth
wheel jaws, and trailer would come loose very
easily).
Section 6 - Combination Vehicles Page 6-8
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Go under trailer and look into the back of the fifth
wheel. Make sure the fifth wheel jaws have closed
around the shank of the kingpin.
Check that the locking lever is in the "lock"
position.
Check that the safety latch is in position over
locking lever. (On some fifth wheels the catch must
be put in place by hand.)
If the coupling isn't right, don't drive the coupled
unit; get it fixed.
Step 14. Connect the Electrical Cord and Check
Air-lines
Plug the electrical cord into the trailer and fasten
the safety catch.
Check both air-lines and electrical line for signs of
damage.
Make sure air and electrical lines will not hit any
moving parts of vehicle.
Step 15. Raise Front Trailer Supports (Landing
Gear)
Use low gear range (if so equipped) to begin
raising the landing gear. Once free of weight,
switch to the high gear range.
Raise the landing gear all the way up. (Never drive
with landing gear only part way up as it may catch
on railroad tracks or other things.)
After raising landing gear, secure the crank handle
safely.
When full weight of trailer is resting on tractor:
Check for enough clearance between rear of tractor
frame and landing gear. (When tractor turns
sharply, it must not hit landing gear.)
Check that there is enough clearance between the
top of the tractor tires and the nose of the trailer.
Step 16. Remove Trailer Wheel Chocks
Remove and store wheel chocks in a safe place.
6.4.2 – Uncoupling Tractor-Semitrailers
The following steps will help you to uncouple safely.
Step 1. Position Rig
Make sure surface of parking area can support
weight of trailer.
Have tractor lined up with the trailer. (Pulling out at
an angle can damage landing gear.)
Step 2. Ease Pressure on Locking Jaws
Shut off trailer air supply to lock trailer brakes.
Ease pressure on fifth wheel locking jaws by
backing up gently. (This will help you release the
fifth wheel locking lever.)
Put parking brakes on while tractor is pushing
against the kingpin. (This will hold rig with pressure
off the locking jaws.)
Step 3. Chock Trailer Wheels
Chock the trailer wheels if the trailer doesn't have
spring brakes or if you're not sure. (The air could
leak out of the trailer air tank, releasing its
emergency brakes. Without chocks, the trailer could
move.)
Step 4. Lower the Landing Gear
If trailer is empty, lower the landing gear until it
makes firm contact with the ground.
If trailer is loaded, after the landing gear makes firm
contact with the ground, turn crank in low gear a few
extra turns. This will lift some weight off the tractor.
(Do not lift trailer off the fifth wheel.) This will:
Make it easier to unlatch fifth wheel.
Make it easier to couple next time.
Step 5. Disconnect Air-lines and Electrical Cable
Disconnect air-lines from trailer. Connect air-line
glad hands to dummy couplers at back of cab or
couple them together.
Hang electrical cable with plug down to prevent
moisture from entering it.
Make sure lines are supported so they won't be
damaged while driving the tractor.
Step 6. Unlock Fifth Wheel
Raise the release handle lock.
Pull the release handle to "open" position.
Keep legs and feet clear of the rear tractor wheels
to avoid serious injury in case the vehicle moves.
Step 7. Pull Tractor Partially Clear of Trailer
Pull tractor forward until fifth wheel comes out from
under the trailer.
Stop with tractor frame under trailer (prevents trailer
from falling to ground if landing gear should collapse
or sink).
Section 6 - Combination Vehicles Page 6-9
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
PAGE 19