Commercial driver’s license (CDL) knowledge tests · California DMV
Warnings, Spring Brakes, ABS and Dual Systems
Questions and answers
-
Is a low air pressure warning signal required on vehicles with air brakes?
Yes, it is required
Every vehicle with air brakes must have a low air pressure warning signal.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
In what tank pressure range must the visible low air pressure warning come on?
Between 55 and 75 psi
A warning you can see must come on when tank pressure falls between 55 and 75 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
On older vehicles, at what pressure must the low air pressure warning come on?
At half the compressor governor cutout pressure
On older vehicles the warning point is one half of the governor cutout pressure.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
What does the low air pressure warning usually look or sound like?
A red light, and a buzzer may also come on
The warning is usually a red light, sometimes with a buzzer.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
What is a “wig wag”?
A low pressure warning device that drops a mechanical arm into your view
The wig wag drops an arm into view when system pressure falls between 55 and 75 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
How is a manual reset wig wag put back out of view?
You place it there by hand, and it stays only when pressure is above 55 psi
The manual reset type must be moved out of view by hand and will not stay there until pressure is above 55 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
At what pressure is it common for the low pressure warning devices on large buses to signal?
80–85 psi
On large buses the warning commonly comes on at 80–85 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗ -
How does the air brake system turn on the brake lights?
With an electric switch that works by air pressure
The stop light switch is an electric switch worked by air pressure.
Section 5 · 5.1.12 – Stop Light Switch ↗ -
Which vehicles may have a front brake limiting valve with a control in the cab?
Some vehicles made before 1975
The manual front brake limiting valve is found on some vehicles built before 1975.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
What does the front brake limiting valve do in the “slippery” position?
Cuts the normal air pressure to the front brakes by half
In the slippery position the valve halves the air pressure going to the front brakes.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
In which position should the front brake limiting valve control be?
“Normal,” to have normal stopping power
The limiting valve reduces stopping power, so keep the control in the normal position.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
What have tests shown about front wheel skids from braking?
They are not likely even on ice
Front wheel braking is good under all conditions, and front wheel skids from braking are unlikely even on ice.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
What is the actual effect of a front brake limiting valve in the “slippery” position?
It reduces the stopping power of the vehicle
Limiting valves were meant to reduce front wheel skids, but they reduce stopping power.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
Automatic front wheel limiting valves reduce air to the front brakes except when what happens?
The brakes are put on very hard, at 60 psi or more application pressure
The automatic valves stop limiting when application pressure reaches 60 psi or more.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
Can the driver control an automatic front wheel limiting valve?
No, the driver cannot control these valves
Automatic limiting valves work on their own.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗ -
Why must emergency and parking brakes be held on by mechanical force?
Because air pressure can eventually leak away
Air can leak away over time, so these brakes must be held by mechanical force, usually springs.
Section 5 · 5.1.14 – Spring Brakes ↗ -
What holds the powerful springs of the spring brakes back while you are driving?
Air pressure
Air pressure holds the springs back. When the air is removed, the springs put the brakes on.
Section 5 · 5.1.14 – Spring Brakes ↗ -
A leak in the air brake system causes all the air to be lost. What happens to the spring brakes?
The springs put the brakes on
Losing all the air lets the springs apply the brakes.
Section 5 · 5.1.14 – Spring Brakes ↗ -
At what air pressure will tractor and straight truck spring brakes come fully on?
In a range of 20 to 45 psi, typically 20 to 30 psi
Spring brakes come fully on when pressure drops to 20 to 45 psi, typically 20 to 30 psi.
Section 5 · 5.1.14 – Spring Brakes ↗ -
The low air pressure warning light and buzzer first come on. What should you do?
Bring the vehicle to a safe stop right away, while you can still control the brakes
Do not wait for the spring brakes to come on automatically. Stop safely while you still have control.
Section 5 · 5.1.14 – Spring Brakes ↗ -
What does the braking power of spring brakes depend on?
The brakes being in adjustment
If the brakes are not adjusted properly, neither the regular brakes nor the emergency and parking brakes work right.
Section 5 · 5.1.14 – Spring Brakes ↗ -
Which vehicles must be equipped with emergency brakes and parking brakes?
All trucks, truck tractors, and buses
All trucks, truck tractors, and buses need emergency and parking brakes.
Section 5 · 5.1.14 – Spring Brakes ↗ -
What does the parking brake control look like in newer vehicles with air brakes?
A diamond-shaped, yellow, push-pull knob
Newer vehicles use a yellow, diamond-shaped push-pull knob for the parking brakes.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
How do you put the parking brakes on with the push-pull control knob?
Pull the knob out
Pull the knob out to apply the parking (spring) brakes and push it in to release them.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
Why should you never push the brake pedal down when the spring brakes are on?
The brakes could be damaged by the combined forces of the springs and the air pressure
Springs plus air pressure can damage the brakes. Not all systems prevent this, so make it a habit not to do it.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
What is a modulating control valve used for?
Applying the spring brakes gradually
A modulating valve lets you apply the spring brakes gradually with a control handle on the dashboard.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
Why does a modulating valve let you apply the spring brakes gradually?
So you can control the spring brakes if the service brakes fail
The more you move the lever, the harder the spring brakes come on, which gives control if the service brakes fail.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
How do you park a vehicle that has a modulating control valve?
Move the lever as far as it will go and hold it in place with the locking device
For parking, the lever goes all the way and is held by the locking device.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
On vehicles with dual parking control valves, such as some buses, what is the separate air tank for?
Releasing the spring brakes so you can move the vehicle in an emergency
When main air pressure is lost and the spring brakes come on, the separate tank can release them so you can move.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
With dual parking control valves, what happens when you release the spring-loaded button?
The spring brakes come on again
The spring brakes stay released only while you hold the control in.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
Why must you plan carefully when moving a vehicle with air from the separate emergency tank?
There is only enough air to release the spring brakes a few times
If the separate supply runs out, you may be stopped in a dangerous place.
Section 5 · 5.1.15 – Parking Brake Controls ↗ -
Truck tractors with air brakes built on or after what date must have anti-lock brakes?
March 1, 1997
ABS is required on air-braked truck tractors built on or after March 1, 1997.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
Air-braked trucks, buses, trailers, and converter dollies built on or after what date must have anti-lock brakes?
March 1, 1998
For air brake vehicles other than truck tractors, the date is March 1, 1998.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
An air-braked bus was built in June 1997. Is it required to have ABS?
No, the requirement for buses starts with those built on or after March 1, 1998
The March 1, 1997 date applies to truck tractors. Buses fall under the March 1, 1998 date.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
What do you check to find the date of manufacture and so determine if your vehicle has ABS?
The certification label
The certification label shows the date of manufacture.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
What is ABS?
A computerized system that keeps your wheels from locking up during hard brake applications
ABS is a computerized system that prevents wheel lockup in hard braking.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
What color are ABS malfunction lamps?
Yellow
Vehicles with ABS have yellow malfunction lamps.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
Where is the ABS malfunction lamp on tractors, trucks, and buses?
On the instrument panel
Tractors, trucks, and buses have the lamp on the instrument panel.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
Where is the ABS malfunction lamp on a trailer?
On the left side, on the front or rear corner
Trailers carry the yellow lamp on the left side, at the front or rear corner.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
On newer vehicles, how does the ABS malfunction lamp behave at start-up?
It comes on for a bulb check and then goes out quickly
The lamp lights briefly at start-up as a bulb check.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
On older ABS systems, how long could the malfunction lamp stay on after start-up?
Until you are driving over 5 mph
On older systems the lamp could stay on until the vehicle is going over 5 mph.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
The ABS lamp stays on after the bulb check, or comes on while you are under way. What may have happened?
You may have lost ABS control at one or more wheels
A lamp that stays on or comes on while driving points to lost ABS control at one or more wheels.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
A towed unit was built before ABS was required. How can you tell if it has ABS?
Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes
On older towed units, look underneath for the electronic control unit and the wheel speed sensor wires.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
When does ABS activate?
Only when wheels are about to lock up
ABS is an addition to your normal brakes and works only when wheels are about to lock.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
How does ABS change your normal braking capability?
It does not decrease or increase it
ABS adds to your normal brakes without changing your normal braking capability.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
What does ABS do for your stopping distance and control?
It does not necessarily shorten stopping distance, but it helps you keep control during hard braking
The benefit of ABS is control during hard braking, not a guaranteed shorter stop.
Section 5 · 5.1.16 – Anti-lock Braking Systems ↗ -
What is a dual air brake system?
Two separate air brake systems that use a single set of brake controls
A dual system has two separate systems, each with its own tanks, hoses, and lines, and one set of controls.
Section 5 · 5.2 – Dual Air Brake ↗ -
In a dual air brake system, what does one system typically operate?
The regular brakes on the rear axle or axles
One system typically works the rear axle brakes and the other works the front axle, and possibly one rear axle.
Section 5 · 5.2 – Dual Air Brake ↗ -
What are the two systems of a dual air brake system called?
Primary and secondary
The first system is the primary system and the other is the secondary system.
Section 5 · 5.2 – Dual Air Brake ↗ -
In a dual air brake system, which system supplies air to the trailer?
Both systems
Both systems supply air to the trailer, if there is one.
Section 5 · 5.2 – Dual Air Brake ↗ -
Before driving a vehicle with a dual air system, how much pressure should the compressor build in both systems?
A minimum of 100 psi
Wait for at least 100 psi in both the primary and secondary systems.
Section 5 · 5.2 – Dual Air Brake ↗ -
The primary gauge reads 100 psi and the secondary gauge reads 70 psi. Are you ready to drive?
No, both systems need at least 100 psi
The minimum of 100 psi applies to both the primary and the secondary system.
Section 5 · 5.2 – Dual Air Brake ↗ -
As pressure builds, the warning light and buzzer shut off at a value set by the manufacturer. What must that value be?
Greater than 55 psi
The manufacturer’s shut-off value for the warning must be greater than 55 psi.
Section 5 · 5.2 – Dual Air Brake ↗ -
In a dual system, when should the warning light and buzzer come on?
Before the air pressure drops below 55 psi in either system
The warning should come on before pressure in either system falls below 55 psi.
Section 5 · 5.2 – Dual Air Brake ↗ -
The low air warning comes on while you are driving a vehicle with a dual air system. What should you do?
Stop right away and safely park the vehicle
Stop right away, park safely, and have the air brake system fixed.
Section 5 · 5.2 – Dual Air Brake ↗ -
One air system of a dual system is very low on pressure. What is the effect?
Either the front or rear brakes will not be operating fully, so stopping takes longer
With one system low, part of the brakes are not fully working and it takes longer to stop.
Section 5 · 5.2 – Dual Air Brake ↗ -
What does a one-way check valve do?
Allows air to flow in one direction only
A one-way check valve lets air pass in a single direction.
Section 5 · One-Way Check Valve ↗ -
Where must the check valve be located on air-brake vehicles?
Between the air compressor and the first reservoir
All air tanks must have a check valve between the compressor and the first reservoir.
Section 5 · One-Way Check Valve ↗ -
The air compressor develops a leak. What does the one-way check valve do?
Keeps air from going out of the tanks
The check valve keeps the stored air from escaping back through a leaking compressor.
Section 5 · One-Way Check Valve ↗
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
- Inspecting Air Brake Systems61
- Using Air Brakes51
- Driving Combination Vehicles Safely50
- Combination Vehicle Air Brakes and ABS59
- 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