CDL Spring and Parking Brake Practice Questions
Spring brakes are the reason a truck that loses all its air comes to a stop instead of rolling away. Every other brake on the vehicle needs air to do its job. The parking and emergency brakes work the other way round. A heavy coiled spring inside the brake chamber does the holding, and air pressure is only what holds that spring back while you are driving. Take the air away, whether by pulling a knob on the dash or by springing a leak at sixty miles an hour, and the spring finally does what it has been pushing to do the whole time.
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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
Spring and parking brakes practice test: quick facts
What this practice test does. Your state's own numbers are cited separately.
- Questions per test
- 5
- Right answers to pass
- 4
- 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 Spring and parking brakes 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
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Scored by subject
You find out which topic is costing you marks, not just a number out of 5.
More about this test
Why the holding force is a spring and not air
A parked vehicle may sit for days. Air bleeds away past seals whether anyone touches it or not, so a parking brake held on by air pressure would quietly let itself off overnight. That is the whole reason the holding force is mechanical, and once you have got that one idea, most of the rest of this subject follows from it.
It explains why the yellow diamond-shaped knob pulls out to apply and pushes in to release. Pulling it out lets air out of the spring brake chambers, and the springs take over. It explains why losing all the system air does the same thing by itself, with nobody touching anything. And it explains why pushing the brake pedal down while the spring brakes are already set is a habit worth losing. The springs and the air are then pressing on the same foundation brake at once, and the combined force can damage it. Plenty of systems are designed to stop that happening. Not all of them are, and the ones that are do not always work.
The three pressures people mix up
Tractor and straight truck spring brakes come fully on somewhere in the 20 to 45 psi range, typically 20 to 30. The figure that gets written into that space by mistake is 55 psi, and 55 is the low air warning, which has to have shown itself before pressure falls that far. The governor's working range of 100 to 125 psi lives at the top of the gauge and has nothing to do with any of this. Warning first, springs much later, governor at the other end entirely.
The gap between the warning and the springs is the only part of this subject that is about driving rather than hardware. It is a short window in which you still pick where the vehicle stops. Sit in it hoping the leak sorts itself out and the springs choose the spot instead, and they have no opinion about bends, live lanes or level crossings. That is why the answer to a low air warning is always to stop now, while the brakes are still yours to use.
Proving it works instead of assuming it
Fan the pressure down deliberately and, somewhere in that 20 to 45 psi band, the tractor protection valve and the parking brake valve should pop out on their own. Watching them pop is how you know the automatic apply still works. Nothing else on the vehicle tells you.
Checking that the parking brake will hold is separate and simpler. Set it, then pull gently against it in a low gear. It either holds or it does not. Do not push the pedal to test it, for the reason above, and do not confuse it with tugging on the slack adjusters, which is a different check done with the parking brakes released.
Adjustment is a parking brake problem too
Spring brakes push on the same foundation brake as the service brakes, so they inherit exactly the same slack. A vehicle whose brakes are out of adjustment has a weak service brake and a weak parking and emergency brake at the same moment. There is no separate backup sitting behind it. That is why adjustment gets checked so often, and why treating it as only a service brake question misses half of what is wrong.
Controls that are not a yellow knob
Older vehicles use a lever, or a round blue knob, in place of the diamond. Some use a modulating control valve, which is built to apply the spring brakes gradually. Parking one of those means moving the lever as far as it will go and holding it there with the locking device. Half way leaves the brakes only partly on, which is not parked. Buses with dual parking control valves carry a separate air tank that can push the spring brakes off again after the main system has gone, but it is a small tank and it will do it only a few times. If you need to move the vehicle, plan the move before you spend the air, or you will end up stopped somewhere worse than where you started.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
What happens if the spring brakes apply while I am driving?
Are spring brakes actually on the real air brakes knowledge test?
Why can I not just hold the vehicle on the brake pedal while parked?
My truck has a lever rather than a yellow knob. Does that change anything?
How do I move a vehicle whose spring brakes are on and whose air is gone?
What is the air brake restriction and does this material affect 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
- Tanks, valves and lines 4 questions, 3 to pass
- Using air brakes on the road 5 questions, 4 to pass
Ready when you are.
5 questions drawn fresh from 65 cited ones. No account, nothing to pay, and every question in the bank is free.