CDL Air Compressor and Governor Practice Questions
The compressor and the governor are the reason there is any air in the tanks to begin with. Everything else in the system spends what those two parts put there. The foot valve spends it, the chambers spend it, a long hill spends a lot of it. When the supply side stops keeping up you rarely find out at once. You find out three or four stops later, when the needle is sitting lower than it was and it is not climbing back, and by then you are already working with less air than you thought you had.
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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
Compressor and governor 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 Compressor and governor 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
What the governor is actually doing
The compressor has no off switch in the cab. It is driven by the engine, through gears or a v-belt, so it turns for as long as the engine turns. Air only builds while the engine is running, which is worth remembering the first time you watch a gauge and wonder why nothing is happening.
The governor is the part that decides whether all that turning does anything. When tank pressure reaches the cut-out figure, around 125 psi, the governor takes the compressor off load and the needle stops rising. Use the brakes and pressure falls. When it reaches the cut-in figure, around 100 psi, the governor puts the compressor back to work.
So the system spends its whole life cycling between those two numbers. That gap is the working reserve, and the reserve is the point. It is the air you have above what an ordinary stop costs, so that normal driving never eats into the margin you would need in a bad moment. A governor cutting in and out at the wrong pressures is not a comfort problem. The reserve may simply not be there when it is needed, which is why a governor that is off the manufacturer's specified pressures has to be fixed before the vehicle goes out.
Cut-out and cut-in, and why people swap them
This is the most mixed-up pair on the air brakes test, and the swap costs people marks on questions they otherwise understand. Cut-out is the top of the range, the higher number, because it is where the compressor gets cut out of the job. Cut-in is the bottom, the lower number, where it is cut back in. Out is up at 125, in is down at 100. Say it that way a few times and the two stop trading places.
Two nearby numbers get dragged in as wrong answers and are worth naming so you can rule them out on sight. 150 psi is not a governor figure at all: that is where the safety relief valve opens, and it is a last-resort protection rather than part of normal cycling. 55 psi is the low air warning, far below anything the governor does in ordinary use. If the gauge is down there, the governor is not the story any more.
Two build-up tests, two sets of numbers
Build-up rate questions give you the figures from the other system and hope you take them. A dual system should build from 85 to 100 psi within 45 seconds at operating rpm. An older pre-1975 single system is allowed 3 minutes to go from 50 to 90 psi at an idle of 600 to 900 rpm. Both the pressure range and the time are different, so learn them as two whole tests rather than four loose numbers. Notice that 85 psi is the bottom of the dual build-up range and has nothing to do with cut-in.
What the compressor asks of you before you drive
Some compressors have their own oil supply instead of being lubricated by the engine. That adds one thing to the routine: check the compressor oil level before driving, because the engine oil check tells you nothing about it. Draining belongs to the tanks at the end of the day, and alcohol belongs to the evaporator, not to the compressor.
If the compressor is belt-driven, the belt goes on the engine compartment check for condition and tightness. A slack or worn belt lets the compressor fall behind demand, and nothing on the dash will say so until pressure is already dropping. The compressor may be air cooled or share the engine cooling system.
Watching the governor work
You can see both figures from the driver's seat. Run the engine at a fast idle and watch the gauge climb and then stop around 125 psi, which is cut-out. Then fan the brake pedal to bleed pressure down and watch the needle start rising again around 100 psi, which is cut-in. Those are two halves of one check, and doing it once on a real vehicle fixes the pair in your head better than any amount of reading.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
What is the difference between cut-out and cut-in pressure?
Does the air compressor run the whole time the engine is running?
Will the low air warning tell me the compressor is falling behind?
Do I check compressor oil on every vehicle?
Are 125 and 100 psi exact figures?
Is the safety relief valve part of the governor?
What does a loose compressor belt do to the system?
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
- Dual air systems 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.