CDL Brake Lag and Stopping Distance Practice Questions
A car's brakes start working the moment your foot moves. Air brakes do not. Push the pedal in a truck and nothing happens at the wheels for half a second or more, because compressed air has to travel down the lines and into a chamber at every wheel before anything grips. Half a second sounds like nothing until you work out what the vehicle does with it. At 55 mph you cover about 32 feet in that gap, with your foot already hard down and the brakes not yet doing a thing. That is road you have given away before the stop has even started, and it is road a car driver never had to give.
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
Brake lag and stopping distance 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 Brake lag and stopping distance 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
Lag is added on, not included
This is the part that catches people, on the test and on the road. In a car, total stopping distance has three parts: perception distance, reaction distance and braking distance. Air brakes make it four. Brake lag distance sits between reaction and braking, and it is extra. It is not hidden inside the other figures and it does not replace any of them. Your perception and your reaction are no different in a truck, because you are the same driver you always were. What is new is the wait while the air gets where it is going.
Keep the order straight and the questions stop looking like tricks. You see the hazard, which is perception. You move your foot, which is reaction. The air travels, which is lag. The brakes slow the vehicle, which is braking.
Know which number belongs to which part
Most wrong answers on this subject are the right number in the wrong slot. At 55 mph, about 32 feet is brake lag distance. About 142 feet is perception distance. About 216 feet is braking distance. Three figures, three different parts of the total, and a question that offers all three at once is testing whether you know which is which, not whether you can remember a number. If you answer 142 feet to a question about lag, you have given a real figure from the wrong part of the sum.
Over 450 feet, from 55 mph
Add the four parts together and an average driver, in a vehicle with air brakes, good brakes and good traction, needs over 450 feet to stop from 55 mph. That is longer than a football field. Stand on one goal line and look at the other: that is what you are asking the road for, and it assumes dry pavement, brakes in adjustment and a driver who is already paying attention. Wet road, worn linings or a heavier load and the figure only grows.
The usual wrong answers are about 200 feet and about 300 feet, and they are wrong in the direction that hurts. A driver who believes 300 feet is enough leaves a following gap sized for a car and discovers the difference at the back of somebody's trailer.
What the numbers are for
Nobody measures a gap in feet at highway speed. The figures matter because they set how much space you have to be carrying before anything goes wrong, and because they explain why a stop that would be routine in a car is not available to you. The half second of lag cannot be trained away, driven around or fixed by pressing harder. The only thing you control is how much room you left, and you have to leave it before you need it.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
Do I have to memorise the stopping distance figures for the test?
Why is 142 feet wrong when the question asks about brake lag?
Why do air brakes have a delay when the brakes on a car do not?
Does the compressor or the governor cause the lag?
Does brake lag get longer when the truck is loaded?
What is the difference between brake lag and brake fade?
Does the delay mean I should brake earlier or pump the pedal?
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
- 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.