Dual Air Brake System Practice Questions
A single air system has one failure that ends a trip badly. A line lets go, the air goes with it, and nothing is holding the vehicle back except the spring brakes coming on by themselves, wherever that happens to be. Modern heavy vehicles are not built that way. They carry two complete air systems behind one brake pedal, so that a failure on one side still leaves you with brakes on the other and enough road to get the vehicle stopped somewhere safe. That is the whole idea, and once you have it the figures stop being a list to memorise and start being consequences of the design.
4 questions, 3 right to pass. Free, untimed, no account.
Correct · A
At roughly what air tank pressure does the governor stop the compressor from pumping any more air?
- About 125 psi Correct answer
- About 100 psi
- About 150 psi
- About 55 psi
Texas Commercial Motor Vehicle Drivers Handbook, 5.1.2 Air Compressor Governor, p. 5-1
Dual air systems practice test: quick facts
What this practice test does. Your state's own numbers are cited separately.
- Questions per test
- 4
- Right answers to pass
- 3
- Time limit
- None
- Cost
- FreeNo account, no card, no limit on retries
- Questions in the bank
- 65Drawn fresh every time you start
- Every answer cited
- YesDocument, section and page on each one
- Last reviewed
- September 2026Re-checked against the source once a year
- Written from
- Texas Commercial Motor Vehicle Drivers Handbook
What this test covers
Every subject below has cited questions behind it. The number is how many are in the bank right now, and each draw is weighted across them.
- Air brake systems 65
How hard the bank is
Every test you start pulls a mix, so you get the easy marks and the ones people actually fail on.
- Straightforward 15
- Moderate 35
- Hard 15
Sample Dual air systems questions
Real questions from the bank, with the answer and the page it came from. Open a few, then take the test properly.
At roughly what air tank pressure does the governor stop the compressor from pumping any more air?
- About 125 psi
- About 100 psi
- About 150 psi
- About 55 psi
Cut-out is around 125 psi. That is where the governor takes the compressor off load and the needle stops climbing. 100 psi is the cut-in figure, 150 psi is where the safety relief valve opens, and 55 psi is the low air warning. Confusing cut-out with cut-in is the classic mistake.
Tank pressure has been dropping as the brakes are used. At about what pressure will the governor let the compressor start pumping again?
- About 100 psi
- About 125 psi
- About 85 psi
- About 55 psi
Cut-in is around 100 psi, cut-out around 125 psi, so the system normally cycles between those two figures. 85 psi is the bottom of the pressure build-up test range and 55 psi is where the low air warning must already have come on.
An air compressor has its own oil supply rather than being lubricated by engine oil. What does that add to the driver's routine?
- Check the compressor oil level before driving
- Drain the compressor at the end of each day of driving
- Top the compressor up with alcohol in cold weather
- Nothing, because the engine oil check already covers it
A compressor with its own oil supply has to have that level checked before driving. Draining at the end of the day is the air tank routine, and the alcohol top-up belongs to the alcohol evaporator, not the compressor.
A governor is not cutting the compressor in and out at the pressures the manufacturer specifies. Why does that have to be fixed before the vehicle goes out?
- It may not keep enough air pressure in the tanks for safe driving
- It stops the brake lights from coming on when the pedal is pushed
- It disables the low air pressure warning signal
- It makes the spring brakes release on their own
The governor is what keeps a working reserve in the tanks. If it cuts in and out at the wrong pressures the reserve may never be there when it is needed. The stop light switch and the low air warning are separate devices, and nothing about a faulty governor releases spring brakes.
The air tanks on a truck have manually operated drain valves. How often does the driver have to drain them?
- At the end of each day of driving
- Once a week
- Before the start of each trip, but not afterwards
- Only when the low air pressure warning comes on
Manual drain valves are the driver's job at the end of every driving day, by a quarter turn or a pull cable, and the tank has to be drained completely. Only automatic drain valves take that job away, and even then the alcohol evaporator still needs checking in cold weather.
Why does the water that collects in the bottom of an air tank matter most in cold weather?
- It can freeze and cause the brakes to fail
- It thins the compressor oil so the compressor seizes
- It sets off the low air pressure warning while driving
- It makes the safety relief valve open below its setting
Compressed air carries water and a little compressor oil, both of which settle in the bottom of the tank. In freezing weather that water can turn to ice and take the brakes out altogether, which is why the tank is drained completely rather than just cracked open.
At about what pressure is the safety relief valve on the first air tank usually set to open?
- 150 psi
- 125 psi
- 100 psi
- 55 psi
The safety relief valve sits in the first tank the compressor pumps to and is usually set for 150 psi, well above the governor's 125 psi cut-out. If it is venting, pressure has gone somewhere it should never reach.
Air is escaping from the safety relief valve on the first air tank. What should the driver do about it?
- Treat it as a fault and have a mechanic repair it
- Nothing, since that is how the valve holds pressure steady
- Drain the tank down and carry on driving
- Reset the governor cut-out setting to a lower pressure
The safety valve is a last line of defence, not a regulator. Air coming out of it means something in the system is letting pressure rise too far, and that is a mechanic's job. Draining the tank or moving the governor setting hides the fault instead of fixing it.
How to use this test
-
Take it cold, once
Do not revise first. You want to know which subjects are actually weak, and a clean run tells you that in ten minutes.
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Read the reason, not the score
The explanation for each answer opens on the next page with its handbook page. That is the bit that makes it stick.
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Retry only what you missed
The result page has a button for exactly that. Far faster than sitting the whole thing again.
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Come back on another day
A different draw each time exposes different gaps. Two runs a week beats ten in one evening.
Why this one is different
You can check every answer
Document, section and page on all of them, plus the date it was read. Nothing goes live without all four.
Nothing is behind a paywall
Every question in the bank is free, every time. There is no premium tier holding the hard ones back.
Scored by subject
You find out which topic is costing you marks, not just a number out of 4.
More about this test
Two systems, one pedal
A dual air brake system is two separate systems, each with its own air tanks, its own hoses and its own lines, feeding one set of brake controls. It is not two pedals. It is not one system for the service brakes and another for the parking brakes. It is not two compressors feeding shared tanks. The split is in the plumbing, not in anything your hands or feet do. One half usually works the rear axles and the other works the front, so whichever one fails, you are left with part of the vehicle braking and part of it not.
The parking brakes are not one of the two halves. The spring brakes sit across the whole system and belong to a different subject.
The numbers, and the two people swap
Three figures carry this subject.
Before you drive, the compressor has to build at least 100 psi in both the primary and the secondary system. Both of them, not the higher one, and not just whichever half feeds the rear brakes.
The low air warning light and buzzer have to come on before pressure in either system falls below 55 psi.
With the engine at operating rpm, the system should build from 85 to 100 psi within 45 seconds.
The swap is 100 and 55. Fifty-five is the level the warning device has to work down to, and it is nowhere near a pressure anyone should drive on. Candidates who answer 55 to the build-up question have remembered a real number and attached it to the wrong question.
There is a fourth item that looks like a figure and is not one. As pressure builds, the warning light and buzzer go out at a value the manufacturer sets, and the only requirement is that it is above 55 psi. It is not exactly 55, and it is not governor cut-out. It also is not permission to move: the light going out and 100 psi in both systems are two separate bars, and the second one is higher. Watch for 3 minutes turning up as a choice too. That belongs to the single system test, which covers a different range at idle.
What you see on the dash
A vehicle with a dual system gives you two supply pressure readings, either as two separate gauges or as one gauge carrying two needles. There is no single combined number, and that is deliberate. A gauge that added the two together, or showed whichever half was higher, would hide the exact failure the dual system exists to survive. Read both, and keep reading them while the pressure builds.
When one half goes
If one system drops very low while you are driving, you still have brakes, but only part of them. Either the front or the rear brakes stop doing their share and the vehicle takes longer to stop. It does not stop itself. The spring brakes do not slam on. The parking brake control does not lock. What happens is quieter than that and worse for it, because the truck simply needs more road than you are expecting. The answer on the test is the answer on the highway: bring it safely to a stop off the road, and get the system repaired before it moves again.
Pulling a trailer
Both halves of the tractor's dual system supply air to the trailer. That is part of why the split is worth having, because trailer braking does not disappear along with one system. The trailer carries its own air tanks, but it has no compressor of its own. Every pound of air it uses came up the line from the tractor.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
How do I tell whether a vehicle has a dual air brake system?
Which half is the primary and which is the secondary?
The warning light went out. Can I release the parking brake and go?
My truck takes longer than 45 seconds to build from 85 to 100 psi. Is it faulty?
If one system fails, can I drive it to a shop?
Does the trailer keep its brakes if one tractor system fails?
Why does the test spend so much time on this one subject?
Still not sure?
Every question names the document and page behind it, so a wrong answer is something you can prove rather than argue about.
Other tests to take
- Chambers, slack adjusters and drums 4 questions, 3 to pass
- Compressor and governor 4 questions, 3 to pass
- Air brake inspection and leak tests 4 questions, 3 to pass
- Brake lag and stopping distance 4 questions, 3 to pass
- Spring and parking brakes 5 questions, 4 to pass
- Tanks, valves and lines 4 questions, 3 to pass
- Using air brakes on the road 5 questions, 4 to pass
Ready when you are.
4 questions drawn fresh from 65 cited ones. No account, nothing to pay, and every question in the bank is free.