Air Brake Tanks, Valves and Lines Questions
Air is not dry. Every time the compressor runs it pushes water vapour and a little compressor oil into the tanks along with the air, and both of them settle at the bottom where nothing blows them back out. Leave them there and one cold night turns that water to ice inside the system, and a driver who walks out to a truck with frozen valves has no brakes at all. That is the whole reason this part of the handbook exists, and it is why nearly everything in it is a job somebody does standing beside the vehicle rather than something they do behind the wheel.
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
Tanks, valves and lines 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 Tanks, valves and lines 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.
-
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.
-
Retry only what you missed
The result page has a button for exactly that. Far faster than sitting the whole thing again.
-
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
Draining the tanks
The question that trips people is simply how often. Tanks with manually operated drain valves get drained at the end of every driving day. Not weekly, not only before a trip, and certainly not only when the low air pressure warning has already come on, which is far too late to be useful. The valve is either a quarter turn or a pull cable, and the tank has to come all the way down, because cracking it open for a second moves air and leaves the water sitting where it was. Automatic drain valves do the job for you and are the only thing that takes it off your list.
The safety relief valve
The first tank the compressor pumps to carries a safety relief valve, and it is usually set for about 150 psi. Put that next to the governor, which cuts the compressor out around 125 psi, and the gap tells you what the valve is for. It is not a regulator and it is not part of normal running. It is the last thing standing between a runaway build-up and a burst tank.
So air coming out of it is never something to live with. It means pressure has reached a level the system should never see, usually because the governor is not cutting out, and the fix is a mechanic. Draining the tank down and carrying on, or winding the governor setting lower to stop the hiss, both hide the fault instead of curing it.
What the tanks hold in reserve
If the compressor quits while you are driving, the tanks still hold enough air to work the brakes several more times. That is a reserve for getting off the road safely, not a fuel gauge you can run down to the next stop, and the wrong answer people pick is usually the pessimistic one, that there is nothing left once the compressor stops.
Alcohol evaporators and tank heaters
An alcohol evaporator feeds alcohol into the air system to cut the chance of ice forming in the valves in cold weather, and its container needs checking and topping up daily when it is cold. It does not touch the water and oil lying in the bottom of the tank, so having one changes nothing about the draining routine. Some tanks with automatic drains also carry an electric heating device, and that is there for one purpose: an automatic drain that has frozen shut cannot drain anything.
Older trucks and the front brake limiting valve
Some older vehicles have a cab control marked "normal" and "slippery" that feeds a front brake limiting valve. In the slippery position it halves the normal air pressure to the front brakes. The idea was to prevent front wheel skids, but front wheel braking is good in every condition and tests show a braking skid on the front wheels is unlikely even on ice, so all the setting really does is throw away stopping power when you are most likely to need it. Leave it in "normal".
Reading the gauge on a long grade
One more thing belongs here because it is read off the same instrument. If holding a steady speed downhill takes more and more application pressure, the brakes are fading and the answer is to slow down and take a lower gear, not to push harder. The same rising reading can also point to brakes out of adjustment, air leaks or something mechanical, and none of those improve with more pedal either.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
Do I still have to drain the tanks if the truck has automatic drains?
How do I actually open a manual drain valve?
Where is the safety relief valve fitted?
Is it normal for a little air to escape from the safety relief valve?
What is the difference between the safety valve and the governor?
Does the alcohol evaporator stop water collecting in the tanks?
My truck has no "normal" and "slippery" control. Have I missed it?
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
- 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
- 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.