Combination air brakes, coupling and inspection
Questions and answers
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What are other names for the trailer hand valve?
The trolley valve or Johnson bar
The trailer hand valve, which works the trailer brakes, is also called the trolley valve or Johnson bar.
Section 6 · 6.2.1 – Trailer Hand Valve ↗PDF, 41 MB -
What should the trailer hand valve be used for?
Only to test the trailer brakes
Use the hand valve only for testing the trailer brakes, never in driving or parking.
Section 6 · 6.2.1 – Trailer Hand Valve ↗PDF, 41 MB -
Which brakes does the foot brake send air to in a combination vehicle?
All of the brakes, including the trailer brakes
The foot brake applies every brake on the rig, which is far less likely to cause a skid or jackknife than the hand valve.
Section 6 · 6.2.1 – Trailer Hand Valve ↗PDF, 41 MB -
Why must you never use the trailer hand valve for parking?
All the air might leak out and unlock the brakes
In trailers without spring brakes the air can leak away and release the brakes, so always use the parking brakes.
Section 6 · 6.2.1 – Trailer Hand Valve ↗PDF, 41 MB -
How do you keep a trailer without spring brakes from moving when you park?
Use wheel chocks
A trailer without spring brakes has no true parking brake, so chock its wheels.
Section 6 · 6.2.1 – Trailer Hand Valve ↗PDF, 41 MB -
What does the tractor protection valve do?
Keeps air in the tractor brake system if the trailer breaks away or leaks badly
If the trailer breaks away or has a bad leak, the tractor protection valve keeps the tractor's air from escaping.
Section 6 · 6.2.2 – Tractor Protection Valve ↗PDF, 41 MB -
What controls the tractor protection valve?
The trailer air supply control valve in the cab
The trailer air supply control in the cab lets you open and shut the tractor protection valve.
Section 6 · 6.2.2 – Tractor Protection Valve ↗PDF, 41 MB -
At about what air pressure will the tractor protection valve close by itself?
20 to 45 psi
When air pressure falls into the 20 to 45 psi range, the tractor protection valve closes automatically.
Section 6 · 6.2.2 – Tractor Protection Valve ↗PDF, 41 MB -
When the tractor protection valve closes, what happens to the trailer?
Air leaves the trailer emergency line and the trailer emergency brakes come on
Closing the valve lets the air out of the trailer emergency line, which puts on the trailer emergency brakes, with possible loss of control.
Section 6 · 6.2.2 – Tractor Protection Valve ↗PDF, 41 MB -
What does the trailer air supply control look like on newer vehicles?
A red eight-sided knob
On newer vehicles, the trailer air supply control is a red, eight-sided knob.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗PDF, 41 MB -
How do you supply the trailer with air using the trailer air supply knob?
Push it in
Push the knob in to supply the trailer with air.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗PDF, 41 MB -
What happens when you pull out the trailer air supply knob?
The air to the trailer is shut off and the trailer emergency brakes come on
Pulling the knob out shuts off the trailer air and applies the trailer emergency brakes.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗PDF, 41 MB -
How may the tractor protection valve control on an older vehicle differ from a newer one?
It may be a lever and may not work automatically
Older controls may be levers with "normal" and "emergency" positions and may not operate automatically.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗PDF, 41 MB -
Which two air-lines does every combination vehicle have?
The service line and the emergency line
Two air-lines run between each vehicle in a combination: the service line and the emergency line.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What are other names for the service air-line?
The control line or signal line
The service line is also called the control line or signal line.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What controls the air in the service line?
The foot brake or the trailer hand brake
The service line pressure changes with how hard you press the foot brake or hand valve.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What do the relay valves connected to the service line do?
Let the trailer brakes be applied more quickly
Relay valves apply the trailer brakes faster than would otherwise be possible.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What is another name for the emergency air-line?
The supply line
The emergency line is also called the supply line.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
Besides controlling the emergency brakes, what does the emergency line do?
Supplies air to the trailer air tanks
The emergency line fills the trailer air tanks and also controls the emergency brakes.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What happens when the emergency line loses air pressure?
The trailer emergency brakes come on
A loss of pressure in the emergency line, for example from a trailer breaking loose, puts on the trailer emergency brakes.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
When the emergency line loses pressure, what happens to the air supply knob?
It pops out
Loss of emergency line pressure also closes the tractor protection valve, so the air supply knob pops out.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What color is often used to code the emergency (supply) line?
Red
Emergency lines are often red so they are not mixed up with the blue service line.
Section 6 · 6.2.4 – Trailer Air-lines ↗PDF, 41 MB -
What color is used for the service lines?
Blue
Blue marks the service lines and red marks the emergency lines.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
What are glad hands?
Couplers that connect the service and emergency air-lines from tractor to trailer
Glad hands couple the tractor's service and emergency air-lines to the trailer.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
What should you do before connecting glad hands?
Clean the couplers and rubber seals
The rubber seals keep the air in, so clean them and the couplers before connecting.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
At what angle do you hold the couplers when connecting glad hands?
90 degrees to each other
Press the two seals together with the couplers at a 90 degree angle, then turn to lock them.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
What happens if you cross the air-lines when coupling?
Supply air goes to the service line instead of charging the trailer air tanks
Crossed lines send supply air to the service line, so the trailer tanks do not charge and the spring brakes do not release.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
The trailer spring brakes do not release when you push the trailer air supply control. What should you check?
The air-line connections
Spring brakes that will not release often mean the air-lines are crossed.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
On an older trailer without spring brakes and with an empty air tank, what can happen if the air-lines are crossed?
You could drive away with no trailer brakes
With no spring brakes and no stored air, crossed lines leave the trailer wheels free, which is very dangerous.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
How do you make sure the trailer brakes work before driving?
Pull gently against them in a low gear
Test with the hand valve or by pulling the air supply control, then pull gently in a low gear.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
What do dummy couplers do when the air-lines are not connected to a trailer?
Keep water and dirt out of the couplers and air-lines
Attaching hoses to dummy couplers keeps the air supply clean.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗PDF, 41 MB -
How are the trailer air tanks filled?
By the emergency (supply) line from the tractor
The supply line from the tractor fills the trailer and dolly air tanks.
Section 6 · 6.2.6 – Trailer Air Tanks ↗PDF, 41 MB -
What sends air from the trailer air tanks to the trailer brakes?
Relay valves
Relay valves send air pressure from the trailer tanks to the brakes.
Section 6 · 6.2.6 – Trailer Air Tanks ↗PDF, 41 MB -
What does the pressure in the service line tell the relay valves?
How much pressure to send to the trailer brakes
Service line pressure, set by the pedal or hand brake, tells the relay valves how hard to apply the trailer brakes.
Section 6 · 6.2.6 – Trailer Air Tanks ↗PDF, 41 MB -
How often should you drain each trailer air tank?
Every day
Water and oil in the tanks can make the brakes fail, so drain each tank every day.
Section 6 · 6.2.6 – Trailer Air Tanks ↗PDF, 41 MB -
Your trailer air tanks have automatic drains. Do you still open the drains?
Yes, to make sure the moisture is out
Automatic drains keep most moisture out, but you should still open the drains to check.
Section 6 · 6.2.6 – Trailer Air Tanks ↗PDF, 41 MB -
What is another name for shut-off valves?
Cut-out cocks
Shut-off valves in the service and supply lines are also called cut-out cocks.
Section 6 · 6.2.7 – Shut-off Valves ↗PDF, 41 MB -
You are pulling doubles. In what position must the shut-off valves at the back of the last trailer be?
Closed
All shut-off valves must be open except those at the back of the last trailer, which must be closed.
Section 6 · 6.2.7 – Shut-off Valves ↗PDF, 41 MB -
Which units are not required to have spring brakes?
Converter dollies and trailers built before 1975
Newer trailers have spring brakes, but converter dollies and pre-1975 trailers do not have to.
Section 6 · 6.2.8 – Trailer Service, Parking and Emergency ↗PDF, 41 MB -
What parking brake does a trailer without spring brakes have?
None
Trailers without spring brakes have emergency brakes that work from stored air, but no parking brake.
Section 6 · 6.2.8 – Trailer Service, Parking and Emergency ↗PDF, 41 MB -
On a trailer without spring brakes, how long will the emergency brakes hold?
Only as long as there is air pressure in the trailer air tank
The air eventually leaks away and then there are no brakes, so chock trailers without spring brakes.
Section 6 · 6.2.8 – Trailer Service, Parking and Emergency ↗PDF, 41 MB -
When may you first notice a major leak in the trailer service line?
When you try to put the brakes on
A service line leak shows up when you brake, and the air loss can drop tank pressure enough to set the emergency brakes.
Section 6 · 6.2.8 – Trailer Service, Parking and Emergency ↗PDF, 41 MB -
Which trailers and converter dollies are required to have ABS?
Those built on or after March 1, 1998
ABS is required on trailers and dollies built on or after March 1, 1998, and many older ones have it too.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗PDF, 41 MB -
What color is the ABS malfunction lamp on a trailer, and where is it?
Yellow, on the left side
Trailers have yellow ABS malfunction lamps on the left side, at the front or rear corner.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗PDF, 41 MB -
On a trailer built before ABS was required, what can you look for under the vehicle to see if it has ABS?
The ECU and wheel speed sensor wires coming from the back of the brakes
Older units may have ABS added, and the ECU and sensor wires underneath show it.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗PDF, 41 MB -
How does ABS change your normal braking capability?
It neither increases nor decreases it
ABS is an addition to your normal brakes and does not change how much braking they can do.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
When does ABS activate?
Only when the wheels are about to lock up
ABS stays out of the way until it senses that a wheel is about to lock.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
What does ABS do for your stopping distance?
It does not necessarily shorten it, but it helps you keep control
ABS helps you keep control during hard braking, but you may not stop any shorter.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
Only the trailer of your rig has ABS. Does that help?
Yes, it still gives you more control during braking
ABS on just the trailer, or even on one axle, still gives more control when braking.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
Only the trailer has ABS and the tractor starts to jackknife. What should you do?
Let up on the brakes, if you can do so safely, until you gain control
Trailer ABS keeps the trailer from swinging out, but a tractor jackknife calls for easing off the brakes.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
How should you brake a tractor-trailer that has ABS?
The same way you always have
With ABS, use only the braking force you need and brake as you normally would.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
Your ABS malfunctions. What brakes do you have?
Your regular brakes still work
A failed ABS leaves normal braking in place. Drive normally and have the system serviced soon.
Section 6 · 6.3.2 – Braking with ABS ↗PDF, 41 MB -
What can happen if the fifth wheel plate is not kept lubricated?
Steering problems from friction between tractor and trailer
Grease the fifth wheel plate as required, or friction between tractor and trailer can make steering hard.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Before coupling, how should the fifth wheel be tilted?
Down toward the rear of the tractor
In the coupling position the wheel tilts down toward the rear, the jaws are open, and the handle is in the automatic lock position.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Before coupling, where should the safety unlocking handle of the fifth wheel be?
In the automatic lock position
With the jaws open and the handle in the automatic lock position, the fifth wheel is ready to couple.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Why should you never back under a trailer at an angle?
You might push the trailer sideways and break the landing gear
Line the tractor up directly in front of the trailer so you do not shove it sideways.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
When you first back toward the trailer, how far do you go?
Until the fifth wheel just touches the trailer
Back slowly until the fifth wheel just touches the trailer, without hitting it.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
How high should the trailer be for coupling?
Low enough that the tractor raises it slightly when backed under it
The tractor should lift the trailer slightly as it backs under.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
What can happen if the trailer is too low when you couple?
The tractor may strike and damage the trailer nose
A trailer that sits too low gets hit in the nose by the tractor.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Why do you shut the engine off after supplying air to the trailer during coupling?
So you can hear the trailer brakes apply and release
With the engine off you can hear the brakes move and the air escape as you apply and release them.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Just before backing under the trailer, what do you do with the air supply knob?
Pull it out to lock the trailer brakes
Lock the trailer brakes by pulling out the air supply knob before you back under.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Which gear do you use to back the tractor under the trailer?
The lowest reverse gear
Back slowly in the lowest reverse gear so you do not hit the kingpin too hard.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
How do you check that the trailer is locked onto the tractor?
Pull the tractor gently forward with the trailer brakes still locked
With the landing gear slightly off the ground, a gentle pull against the locked trailer brakes tests the connection.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Why do you take the key with you before going under the trailer to inspect the coupling?
So someone else will not move the truck while you are under it
Shut off the engine and keep the key so no one can move the truck while you are underneath.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
After coupling you see space between the upper and lower fifth wheel. What might be wrong?
The kingpin may be on top of the closed fifth wheel jaws
Any space means something is wrong, and the trailer could come loose very easily.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Looking into the back of the fifth wheel, what must the jaws be closed around?
The shank of the kingpin
The jaws must lock around the shank of the kingpin, not its head.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Where must the safety latch be after coupling?
In position over the locking lever
The locking lever must be in "lock" with the safety latch over it. On some fifth wheels you place the catch by hand.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
The coupling is not right. What do you do?
Do not drive the coupled unit and get it fixed
Never drive a rig that is not coupled correctly.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
When raising the landing gear, when do you switch from the low gear range to the high gear range?
Once the landing gear is free of weight
Start in the low gear range and switch to high once the gear is free of weight.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
What can happen if you drive with the landing gear only part way up?
It may catch on railroad tracks or other things
Raise the landing gear all the way so it cannot catch on tracks or other objects.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Why must there be enough clearance between the rear of the tractor frame and the landing gear?
So the tractor does not hit the landing gear in a sharp turn
With the full weight on the tractor, check that a sharp turn will not bring the frame into the landing gear.
Section 6 · 6.4.1 – Coupling Tractor-Semitrailers ↗PDF, 41 MB -
Before uncoupling, what must you check about the parking area?
That its surface can support the weight of the trailer
The ground must hold the trailer's weight on its landing gear.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
Why should the tractor be lined up with the trailer when uncoupling?
Pulling out at an angle can damage the landing gear
A straight pull-out protects the landing gear.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
How do you ease the pressure on the fifth wheel locking jaws when uncoupling?
Back up gently with the trailer brakes locked
Backing gently against the locked trailer takes pressure off the jaws so the release lever moves easily.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
When uncoupling, when should you chock the trailer wheels?
If the trailer does not have spring brakes or you are not sure
Air in a trailer without spring brakes can leak away and release its emergency brakes.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
When uncoupling a loaded trailer, what do you do after the landing gear makes firm contact with the ground?
Turn the crank in low gear a few extra turns
A few extra turns lift some weight off the tractor, making it easier to unlatch and to couple next time, without lifting the trailer off the fifth wheel.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
How should you hang the electrical cable after disconnecting it?
With the plug down
Hanging it plug down keeps moisture out.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
When you pull the fifth wheel release handle, what should you keep clear of?
The rear tractor wheels
Keep your legs and feet away from the rear tractor wheels in case the vehicle moves.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
Why do you stop with the tractor frame still under the trailer during uncoupling?
It keeps the trailer from falling if the landing gear collapses or sinks
Leaving the frame under the trailer catches it if the landing gear gives way.
Section 6 · 6.4.2 – Uncoupling Tractor-Semitrailers ↗PDF, 41 MB -
Before uncoupling a pintle hook, where should you park the trailer?
On a firm level surface, with the trailer tires blocked
Park on firm level ground and block the trailer tires before you uncouple.
Section 6 · 6.4.4 - Uncoupling a Pintle Hook ↗PDF, 41 MB -
Which trailers are required by law to have safety chains?
All trailers
Every trailer must have safety chains, and you attach them when coupling.
Section 6 · 6.4.7 - Coupling a Gooseneck Hitch ↗PDF, 41 MB -
When coupling a drawbar, the safety cover bar will not lodge into its seat. What is the rule?
Travel is strictly forbidden
If the safety cover bar does not seat properly, the rig must not travel.
Section 6 · 6.4.5 - Coupling a Drawbar ↗PDF, 41 MB -
What inspection procedure do you use on a combination vehicle?
The seven-step procedure from Section 2, plus the added combination checks
Use the seven-step method and add the checks that apply to combinations.
Section 6 · 6.5 – Inspecting a Combination Vehicle ↗PDF, 41 MB -
During the walk-around, how should the locking jaws grip the kingpin?
Around the shank, not the head
Jaws around the head of the kingpin mean a bad coupling.
Section 6 · 6.5.1 – Additional Things to Check during a Walk-around Inspection ↗PDF, 41 MB -
How should the air-lines to the trailer be secured?
Properly connected with no leaks and with enough slack for turns
Air-lines need enough slack to follow the trailer in turns.
Section 6 · 6.5.1 – Additional Things to Check during a Walk-around Inspection ↗PDF, 41 MB -
What do you check about the position of a sliding fifth wheel?
That it is not so far forward that the tractor frame hits the landing gear in turns
Too far forward, the frame can hit the landing gear or the cab can hit the trailer during turns.
Section 6 · 6.5.1 – Additional Things to Check during a Walk-around Inspection ↗PDF, 41 MB -
In the air flow check, what does hearing air escape from the emergency line at the rear of the last trailer show?
The entire system is charged
Air escaping at the last trailer proves the emergency line is charged all the way back.
Section 6 · 6.5.2 – Combination Vehicle Brake Check ↗PDF, 41 MB -
If the tractor protection valve does not work right, what can a trailer brake leak do?
Drain all the air from the tractor
Without a working tractor protection valve, a trailer leak can empty the tractor's air and set the emergency brakes.
Section 6 · 6.5.2 – Combination Vehicle Brake Check ↗PDF, 41 MB -
In the trailer emergency brake test, what do you check before pulling out the air supply control?
That the charged trailer rolls freely
First make sure the charged trailer rolls freely, then stop, pull the control out and pull gently to feel the brakes hold.
Section 6 · 6.5.2 – Combination Vehicle Brake Check ↗PDF, 41 MB -
In normal driving, how should the trailer service brakes be controlled?
With the foot pedal
Test the trailer brakes with the hand valve, but use the foot pedal in normal operation.
Section 6 · 6.5.2 – Combination Vehicle Brake Check ↗PDF, 41 MB
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