CDL Brake Chamber and Foundation Brake Practice Questions
Air brakes are a way of sending a message down a long vehicle. The pedal is the message and the air is only how it travels. At every wheel there is a brake chamber bolted near the axle, and that chamber is where the message finally turns into something physical. The compressor, the governor, the tanks, the valves and the gauge you are taught to watch all exist to put usable air behind that chamber at the moment you ask for it. If the mechanical end is worn, out of adjustment or leaking, nothing upstream will stop the truck for you.
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
Chambers, slack adjusters and drums 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 Chambers, slack adjusters and drums 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
That is why this is worth learning properly even though it is a short subject. Once you can follow the chain from your foot to the drum, the numbers on the rest of the air brakes test stop being a list to memorise and start being descriptions of a machine you understand.
From the pedal to the drum
Follow the order, because the questions follow the order. You press the pedal. Air goes into the brake chamber. The chamber pushes a rod out. The push rod moves the slack adjuster, which is a lever clamped to the end of the brake camshaft. Turning that shaft turns the s-cam at the far end, and the s-cam forces the two brake shoes apart and against the inside of the drum, which turns with the wheel. Chamber, push rod, slack adjuster, camshaft, s-cam, shoes, drum. Say it in that order a few times and a good share of this subject is done.
Now lift off. The cam rotates back, but the cam is not what pulls the shoes in. A spring does that work, and it is the part people forget, because nothing pushes the shoes off the drum with air. A broken return spring leaves that brake dragging on the drum and cooking itself while the driver feels nothing wrong through the pedal.
The two brakes that are not s-cam
Most drum brakes on the road are s-cam, and most questions assume it. The handbook describes two other arrangements, and the test likes them because they sound alike on paper.
A wedge brake has no cam and no slack adjuster. The push rod drives a wedge straight in between the ends of the two brake shoes and shoves them apart. Some wedge brakes have a single chamber, some have two, pushing wedges in at both ends of the shoes. They may be self-adjusting or they may need adjusting by hand.
An air-operated disc brake keeps the chamber and the slack adjuster, which is exactly where people go wrong. Instead of a cam, the slack adjuster turns a power screw, and the screw clamps the disc between the lining pads of a caliper, in the way a large c-clamp squeezes. So a question that mentions a slack adjuster has not told you the brake is an s-cam. Both disc and wedge brakes are less common than s-cam drum brakes.
Heat is the real limit
Shoes rubbing the inside of a drum turn motion into heat, and a drum can only take so much of it. How much depends on how hard the brakes have been worked and for how long, which is why brakes that feel fine around town can give up on a long downgrade. Past that point they simply stop working. That is brake fade, and nothing about it shows on the air pressure gauge.
The same thinking explains a habit worth breaking. Pressing and releasing the pedal when there is no need for it wastes air, because every release vents some out of the system, and it can go out faster than the compressor puts it back. Let pressure fall far enough and the brakes stop working for a different reason entirely. Fanning the pedal belongs in the pressure tests, not on the road.
What you actually look at
Brake chambers get checked on the walk-around, at the same time as the drums, the linings and the rest of the mechanical parts. On the air hoses running out to each chamber you are looking for cuts, and for wear where a hose has been rubbing against something. A chafed hose is a leak that has not happened yet, and a leak at the chamber costs you that wheel's brake.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
What does the slack adjuster actually do?
Do I need to know wedge and air disc brakes, or just s-cam?
Why are there only seven questions on this subject?
What is the difference between brake fade and a dragging brake?
What do I check on the brake chambers during a pre-trip inspection?
Are spring brakes part of this 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
- 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
- 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.