Commercial driver’s license (CDL) knowledge tests · California DMV
Combination Vehicle Air Brakes and ABS
Questions and answers
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What are other names for the trailer hand valve?
Trolley valve or Johnson bar
The trailer hand valve is also called the trolley valve or Johnson bar.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
What should the trailer hand valve be used for?
Only to test the trailer brakes
The hand valve works the trailer brakes and should be used only to test them.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
Why should you not use the trailer hand valve in driving?
Because of the danger of making the trailer skid
Braking the trailer alone can make it skid. The foot brake carries much less risk of a skid or jackknife.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
Which brakes does the foot brake send air to?
All of the brakes on the vehicle, including the trailer or trailers
The foot brake applies every brake on the vehicle.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
Why must you never use the hand valve for parking?
All the air might leak out, unlocking the brakes on trailers without spring brakes
Always use the parking brakes when parking, because air held by the hand valve can leak away.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
Your trailer does not have spring brakes. How do you keep it from moving when parked?
Use wheel chocks
A trailer without spring brakes must be held with wheel chocks.
Section 6 · 6.2.1 – Trailer Hand Valve ↗ -
What does the tractor protection valve do?
Keeps air in the tractor or truck brake system if the trailer breaks away or develops a bad leak
The valve protects the tractor’s air supply when the trailer breaks away or leaks badly.
Section 6 · 6.2.2 – Tractor Protection Valve ↗ -
What controls the tractor protection valve?
The “trailer air supply” control valve in the cab
The trailer air supply control in the cab opens and shuts the tractor protection valve.
Section 6 · 6.2.2 – Tractor Protection Valve ↗ -
At what air pressure will the tractor protection valve close automatically?
In the range of 20 to 45 psi
The valve closes by itself when pressure is low, in the 20 to 45 psi range.
Section 6 · 6.2.2 – Tractor Protection Valve ↗ -
What happens when the tractor protection valve closes?
It stops air going out of the tractor and lets the air out of the trailer emergency line
Closing the valve keeps the tractor’s air and empties the trailer emergency line, so the trailer emergency brakes come on.
Section 6 · 6.2.2 – Tractor Protection Valve ↗ -
What risk comes with the trailer emergency brakes coming on when the tractor protection valve closes?
Possible loss of control
The trailer emergency brakes coming on can cause loss of control.
Section 6 · 6.2.2 – Tractor Protection Valve ↗ -
What does the trailer air supply control look like on newer vehicles?
A red 8-sided knob
The trailer air supply control is a red 8-sided knob.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
How do you supply the trailer with air using the trailer air supply control?
Push the knob in
Push in to supply air. Pull out to shut the air off and put on the trailer emergency brakes.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
What happens when you pull the trailer air supply knob out?
The air is shut off and the trailer emergency brakes are put on
Pulling the knob shuts off the trailer’s air and applies its emergency brakes.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
When will the trailer air supply valve pop out by itself?
When air pressure drops into the range of 20 to 45 psi
The knob pops out, closing the tractor protection valve, at 20 to 45 psi.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
How may tractor protection valve controls on older vehicles differ?
They may not operate automatically, and there may be a lever rather than a knob
Older “emergency” valves may be manual levers that do not pop out on their own.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
On an older vehicle with a lever, which position is used for pulling a trailer?
“Normal”
Normal is for pulling a trailer. Emergency shuts the air off and puts on the trailer emergency brakes.
Section 6 · 6.2.3 – Trailer Air Supply Control ↗ -
How many air lines does every combination vehicle have, and what are they?
Two: the service line and the emergency line
The service and emergency lines run between each vehicle in the combination.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What are other names for the service air line?
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 ↗ -
What controls the air carried by the service line?
The foot brake or the trailer hand brake
Pressure in the service line changes with how hard you press the foot brake or hand valve.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What do the relay valves connected to the service line do?
Allow the trailer brakes to be applied more quickly than would otherwise be possible
Relay valves speed up the application of the trailer brakes.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What is another name for the emergency air line?
Supply line
The emergency line is also called the supply line.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What are the two purposes of the emergency air line?
It supplies air to the trailer air tanks and controls the emergency brakes on combination vehicles
The emergency line fills the trailer tanks and controls the trailer emergency brakes.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What happens when air pressure in the emergency line is lost?
The trailer emergency brakes come on and the tractor protection valve closes
Loss of emergency line pressure applies the trailer emergency brakes and makes the air supply knob pop out.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
What could cause a loss of pressure in the emergency line?
A trailer breaking loose and tearing apart the emergency air hose
A breakaway, or a hose, metal tubing, or other part breaking, lets the air out.
Section 6 · 6.2.4 – Trailer Air Lines ↗ -
With which color are emergency lines often coded?
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 ↗ -
What are glad hands?
Coupling devices that connect the service and emergency air lines from the truck or tractor to the trailer
Glad hands are the hose couplers for the service and emergency air lines.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
What is the rubber seal on a glad hand for?
It prevents air from escaping
The rubber seal stops air from leaking at the coupling.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
What should you do to the couplers and rubber seals before making a connection?
Clean them
Clean the couplers and rubber seals before connecting.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
At what angle to each other do you press the two glad hand seals together?
90 degrees
Press the seals together with the couplers at 90 degrees, then turn the glad hand on the hose to join and lock them.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
Which color is used for the service lines?
Blue
Blue marks the service lines and red marks the emergency (supply) lines.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
Besides colors, what is sometimes used to tell the air lines apart?
Metal tags stamped “service” and “emergency”
Metal tags with the words service and emergency are sometimes attached to the lines.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
What happens if you cross the air lines on a trailer with spring brakes?
Supply air goes to the service line, so the trailer tanks are not charged and the spring brakes will not release
With crossed lines, air is not available to release the trailer spring brakes.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
You push in the trailer air supply control and the trailer spring brakes do not release. What should you check?
The air line connections
Spring brakes that will not release may mean the air lines are crossed.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
You cross the air lines on an older trailer without spring brakes. What is the danger?
You can drive away, but you will not have trailer brakes
Without spring brakes the wheels turn freely, so you could drive off with no trailer brakes.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
How do you test the trailer brakes before driving?
Apply them with the hand valve or by pulling the air supply control, then pull gently against them in a low gear
Always test the trailer brakes before driving by pulling gently against them in a low gear.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
What are “dead end” or dummy couplers for?
Holding the hoses when not in use, to keep water and dirt out of the coupler and air lines
Use the dummy couplers when the air lines are not connected to a trailer.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
There are no dummy couplers on your tractor. What can sometimes be done with the glad hands?
They can be locked together
Depending on the couplings, the glad hands can sometimes be locked together to keep the air supply clean.
Section 6 · 6.2.5 – Hose Couplers (Glad Hands) ↗ -
What fills the air tanks on a trailer or converter dolly?
The emergency (supply) line from the tractor
Each trailer and converter dolly has one or more air tanks, filled through the emergency line.
Section 6 · 6.2.6 – Trailer Air Tanks ↗ -
What sends air pressure from the trailer air tanks to the trailer brakes?
Relay valves
Relay valves send air from the tanks to the brakes.
Section 6 · 6.2.6 – Trailer Air Tanks ↗ -
What tells the relay valves how much pressure to send to the trailer brakes?
The pressure in the service line
Service line pressure, set by the brake pedal and trailer hand brake, tells the relay valves how much to send.
Section 6 · 6.2.6 – Trailer Air Tanks ↗ -
How often should you drain each trailer air tank?
Every day
Water and oil must not build up in the tanks, so drain each tank daily.
Section 6 · 6.2.6 – Trailer Air Tanks ↗ -
Your trailer tanks have automatic drains. What should you still do?
Open the drains to make sure
Automatic drains keep most moisture out, but you should still open the drains to be sure.
Section 6 · 6.2.6 – Trailer Air Tanks ↗ -
Where are shut-off valves found?
In the service and supply air lines at the back of trailers used to tow other trailers
Shut-off valves let the air lines be closed when no other trailer is being towed.
Section 6 · 6.2.7 – Shut-Off Valves ↗ -
Which shut-off valves must be closed?
The ones at the back of the last trailer
All shut-off valves are open except those at the back of the last trailer.
Section 6 · 6.2.7 – Shut-Off Valves ↗ -
You are pulling two trailers. In what position are the shut-off valves at the back of the first trailer?
Open
Only the valves at the back of the last trailer are closed, so air reaches every trailer.
Section 6 · 6.2.7 – Shut-Off Valves ↗ -
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 need not.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
On trailers without spring brakes, what do the emergency brakes work from?
The air stored in the trailer air tank
Their emergency brakes use the air stored in the trailer tank.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
What kind of parking brake do trailers without spring brakes have?
None
These trailers have no parking brake. The emergency brakes come on when the air supply knob is pulled or the trailer is disconnected.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
Why is it very important to use wheel chocks when you park trailers without spring brakes?
The emergency brakes hold only as long as there is air in the trailer tank, and the air eventually leaks away
When the trailer tank air leaks away there are no brakes.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
What does a major leak in the emergency line cause?
The tractor protection valve closes and the trailer emergency brakes come on
A big emergency line leak closes the tractor protection valve and applies the trailer emergency brakes.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
When might you first notice a major leak in the service line?
When you try to put the brakes on
A service line leak shows only when you brake. Then air is lost quickly, and the trailer emergency brakes may come on.
Section 6 · 6.2.8 – Trailer Service, Parking, and ↗ -
Trailers and converter dollies built on or after what date are required to have ABS?
March 1, 1998
All trailers and converter dollies built on or after March 1, 1998 must have ABS.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗ -
Where do you look for the yellow ABS malfunction lamp on a trailer?
On the left side, on the front or rear corner
Trailers have the yellow ABS lamp on the left side, at the front or rear corner.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗ -
A trailer was built before ABS was required and you cannot tell if it has ABS. What do you look for?
The ECU and wheel speed sensor wires coming from the back of the brakes, under the vehicle
Many older trailers were voluntarily fitted with ABS. Look underneath for the control unit and sensor wires.
Section 6 · 6.3.1 – Trailers Required to Have ABS ↗ -
Only your trailer has ABS. What does that change?
The trailer is less likely to swing out
Trailer ABS makes a trailer swing less likely, but you must still let up on the brakes if you lose steering or the tractor starts to jackknife.
Section 6 · 6.3.2 – Braking With ABS ↗ -
What will ABS NOT allow you to do?
Drive faster, follow more closely, or drive less carefully
ABS helps with control and wheel lockup. It is not a reason to drive faster, follow more closely, or take less care.
Section 6 · 6.3.2 – Braking With ABS ↗ -
As you slow a tractor-trailer with ABS, what should you do?
Monitor the tractor and trailer and back off the brakes, if it is safe, to stay in control
Watch both units while slowing and ease off the brakes if needed to keep control.
Section 6 · 6.3.2 – Braking With ABS ↗ -
The ABS on your combination malfunctions. What brakes do you have?
Regular brakes, so drive normally and get the system serviced soon
A failed ABS leaves your regular brakes working.
Section 6 · 6.3.2 – Braking With ABS ↗
Questions and answers by topic
- Licenses, endorsements and tests105
- Disqualifications and state rules110
- Vehicle inspection80
- Basic control and shifting gears47
- Seeing and communicating46
- Controlling your speed62
- Managing space40
- Seeing hazards49
- Distracted and Aggressive Driving71
- Night, Fog, Winter and Hot Weather124
- Railroad Crossings and Mountain Driving65
- Emergencies, ABS and Skids102
- Accident Procedures and Fires49
- Alcohol, Drugs and Illness41
- Hazardous Materials Rules for All Drivers42
- Transporting Cargo Safely79
- Bus Inspection, Loading and Hazardous Materials74
- Driving a Bus: Stops, Crossings and Prohibited Practices56
- Air Brake Parts: Compressor, Tanks and Foundation Brakes52
- Warnings, Spring Brakes, ABS and Dual Systems59
- Inspecting Air Brake Systems61
- Using Air Brakes51
- Driving Combination Vehicles Safely50
- Coupling and Uncoupling63
- Inspecting a Combination Vehicle24
- Pulling Double and Triple Trailers16
- Coupling, Inspecting and Brake Checks for Doubles34
- Tank Vehicles62
- HazMat basics and who does what48
- Hazard classes, labels and lists64
- Shipping papers and package markings49
- Placards and placard tables40
- Loading and unloading HazMat58
- Bulk packaging and tanks30
- HazMat driving and parking rules72
- HazMat emergencies and reporting60
- Danger zones and mirrors34
- Loading and unloading students47
- Emergency exit and evacuation34
- Railroad-highway crossings40
- Student management, ABS and special situations47