258 questions · General Knowledge + Endorsements · Free
258 practice questions for the California CDL knowledge test, written from Sections 2 through 10 of the California Commercial Driver Handbook: driving safely, cargo, passengers, air brakes, combination vehicles, doubles and triples, tank vehicles, hazardous materials, and school buses. Every answer comes with a plain English explanation, a note on why each wrong choice fails, and the handbook page it comes from. Free, no account required.
This is practice for the California CDL knowledge test. Every question is based on Sections 2 through 10 of the California Commercial Driver Handbook (DL 650 (2023)), and each explanation cites the page it comes from. These are nine sections of the commercial handbook, so keep studying the California permit practice test and the rest of the commercial handbook alongside it.
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The questions here are practice, not the official test, and they cover Sections 2 through 10 of the handbook: general driving safety, cargo, passengers, air brakes, combination vehicles, doubles and triples, tank vehicles, hazardous materials, and school buses. Check the California licensing agency before your appointment for the current test format and fees.
Source: California Commercial Driver Handbook (DL 650 (2023), California DMV). Downloaded and checked 2026-10-14.
Every question below comes from the California CDL question bank on this page. Open a question to see the correct answer and the explanation. Nothing here is hidden behind a signup.
1. How many braking systems make up a complete air brake system?
Three: service, parking, and emergency. Air brakes combine three systems that share hardware but do different jobs. The service system works from the brake pedal in normal driving, the parking system holds the vehicle when parked, and the emergency system borrows parts of the other two to stop the vehicle after a failure. Knowing which system does what explains why a truck can lose pedal braking yet still stop, and why Section 5 treats a parking brake control as safety equipment rather than a convenience. Expect test questions to describe one system and ask which of the three it is.
2. Tank pressure falls while you drive. At about what pressure should the compressor start pumping again?
About 100 psi. Cut-in is the bottom of the governor's working band: when tank pressure sinks to roughly 100 psi, the governor lets the compressor resume pumping so normal braking never comes close to draining the reservoirs. If your gauge keeps sliding below that point with the engine running, the compressor, its drive, or the governor is not keeping up, and every brake application brings the low warning closer. Treat a gauge that will not recover toward cut-out as a defect to fix before departure, not as a quirk to monitor hopefully from the driver's seat.
3. What is the alcohol evaporator for?
Putting alcohol into the air to reduce ice forming in valves. The evaporator meters alcohol into the air stream so moisture inside valves and lines is less likely to freeze into a plug during cold weather. It is a supplement, not a substitute: the handbook still expects daily tank drainage to remove water and oil unless automatic drains are fitted. The container needs checking and filling every day in cold weather, because an empty evaporator protects nothing. Drivers who understand the pairing, alcohol against ice in the lines and drains against water in the tanks, keep both habits instead of choosing one.
4. What do the supply pressure gauges tell you?
How much pressure is in the air tanks. Supply gauges read the reservoirs themselves, one for a single system or one per half, sometimes as two needles in one dial, on a dual system. They are your fuel gauge for braking: cut-in, cut-out, warnings, and spring brake behaviour all announce themselves there first. The application gauge, by contrast, only shows what a pedal press is asking for. A driver who glances at supply pressure before rolling, and again whenever braking feels different, catches compressor and leak problems while they are still announcements rather than emergencies on a grade.
5. What turns on the brake lights when you apply air brakes?
An electric switch that works by air pressure. The brake lights are driven by an electric switch operated by air pressure, so the lamps light whenever the system applies, whether the push came from your foot or another air control. That detail matters during inspection: lights that lag or stay dark can point to the switch or its air supply rather than bulbs. Drivers behind you plan around those lamps, especially on grades where your speed changes without much visible pitch. Confirming the lights respond during the brake check closes the loop between what your foot does and what following traffic sees.
6. How do you apply the parking brakes on a newer air brake vehicle?
Pull out the diamond-shaped yellow push-pull knob. The parking brake control on newer vehicles is the diamond-shaped yellow knob: pull it out and air leaves the spring brake chambers so the springs apply; push it in and air holds them off again. Colour and shape are deliberate, findable by touch in a dark or smoky cab. Older vehicles may use a lever instead, and buses add their own variations, so the pre-trip habit is to locate and work your vehicle's control before you need it in a hurry. On the skills test, reaching for the wrong knob announces itself instantly.
7. What makes a dual air brake system dual?
Two separate air systems sharing one set of controls. Dual means redundancy in supply, not in the driver's limbs. Two independent air systems, each with its own tanks, hoses, and lines, feed the brakes through a single set of controls, one typically working the rear axles and the other the front. Lose one side and the other still stops the vehicle, just more slowly. That architecture explains the paired gauges on the dash and why the inspection treats each half separately. It also explains why a single low warning on a dual vehicle is a stop-now event rather than a shrug: half your braking just resigned.
8. During the engine compartment check, what do you inspect on a belt-driven air compressor?
The condition and tightness of the belt. A belt-driven compressor is only as reliable as the belt turning it, so Step 2 of the inspection puts your hands on that belt: sound condition, no cracking or fraying, and tight enough not to slip under load. A belt that lets go takes the whole air supply with it, and the tanks buy you minutes, not a shift. Gear-driven compressors skip this item, which is why the check is written conditionally. Catching a glazed or loose belt in the yard is a five-minute fix compared with losing pressure on a grade with a load behind you.
9. In the applied leakage test, how much pressure drop is allowed in one minute for a combination vehicle?
No more than 4 psi. The applied test starts at governor cut-out with the engine off, parking brakes released, and the foot brake fully held for a minute. A single vehicle may lose no more than 3 psi in that minute; a combination vehicle, with more chambers and connections, is allowed 4. Exceed either figure and something leaks enough to need repair before the vehicle works. This is one of the starred in-cab tests on the skills exam, where you must also say the limit for the vehicle in front of you, not merely recite both numbers and hope.
10. On a dual air system, what pressure buildup rate must the compressor show at operating idle?
85 to 100 psi within 45 seconds. The buildup check times recovery through the working band: at a normal operating idle of roughly 600 to 900 rpm, a dual system should climb from about 85 to about 100 psi within 45 seconds. Slower means the compressor, its drive, or its governor is losing a race the brakes will eventually ask it to win. The older single-system standard, 50 to 90 within three minutes, belongs to pre-1975 equipment and makes a tidy wrong answer. Verbalize both the window and the verdict for your vehicle, because the examiner grades the sentence as much as the stopwatch.
11. In stab braking, why wait for the wheels to start rolling before reapplying?
If you reapply before the wheels roll, the vehicle will not straighten out. A locked wheel has no direction in it; it slides wherever momentum points it. Stab braking therefore works in beats: full application, release, and then patience for up to a second while the wheels spin back up and bite, because only a rolling wheel can pull the vehicle straight again. Reapply a breath early and you lock a wheel that was about to save you. That single second is the hardest part of the technique under fear, which is why the handbook spells it out as a number instead of leaving it as advice about feel.
12. On a long downgrade in the right gear, how do you use the service brakes?
Apply until you are about 5 mph below safe speed, release, and repeat as speed climbs back. The technique is sometimes called snubbing: let speed rise to your safe number, brake firmly enough to feel a definite slowdown down to about 5 mph under it, roughly three seconds of application, then release completely and let the brakes cool while the engine holds the grade. Repeat as needed to the bottom. Steady light dragging, the instinctive alternative, keeps drums hot without rest and manufactures fade. The gear chosen before the descent does most of the work; the brakes only trim. If the trimming starts lengthening, the gear choice was wrong and a stop to change it beats a prayer.
13. How much more likely is a fully loaded rig to roll over in a crash than an empty rig?
10 times more likely. A loaded rig carries its weight high, so the center of gravity sits farther from the road and the vehicle tips more easily once a turn or swerve starts. The handbook puts the difference at 10 times for fully loaded rigs compared with empty ones. That is why rollover prevention is mostly about speed on curves and ramps: enter slowly enough that you never need a sudden steering correction, and treat a top-heavy load as a warning to slow down even earlier than you would in a single vehicle.
14. When do large combination vehicles take longer to stop?
When empty, compared with fully loaded. It surprises many new drivers, but an empty combination vehicle can need more distance to stop than a loaded one. Lightly loaded rigs ride on very stiff suspension springs with strong brakes, a mix that gives poor traction and locks wheels easily. A locked trailer can swing out, and a tractor can jackknife quickly. Brake early, allow generous following distance, and never count on an empty rig stopping short just because it weighs less.
15. What is the earliest and best way to recognize that a trailer has started to skid?
Seeing the trailer swing in your mirrors. By the time a trailer skid reaches your hands or ears, the trailer may already be out of its lane. The handbook's earliest warning is visual: check your mirrors whenever you brake hard and watch whether the trailer is staying where it belongs. Catching the first small swing gives you the chance to release the brakes and let the trailer wheels grip again. Once the trailer has swung wide, preventing a jackknife becomes very difficult, which is why mirror checks belong in every hard stop.
16. If you cannot finish a turn without entering another traffic lane, when should you turn wide?
Turn wide as you complete the turn. Swinging left before a right turn opens a gap on your right that other drivers will try to use, and your trailer can then sweep over the vehicle that filled it. The handbook's safer pattern is to keep the rear of the rig close to the curb and, if the turn needs more room, take it late: turn wide as you complete the turn. That timing keeps the right side blocked while your trailer is off-tracking inward, protecting both curbside space and anyone beside you.
17. What does the tractor protection valve protect?
The tractor brake system, keeping its air if the trailer breaks away or leaks badly. If a trailer breaks away or its air lines tear open, the tractor could bleed all of its air through the open lines and lose its own brakes at the worst possible moment. The tractor protection valve prevents that: it keeps air in the tractor or truck brake system and closes off the trailer lines. The driver controls it with the trailer air supply valve in the cab, and it also closes on its own when pressure falls too low. This one valve is why a trailer emergency does not have to become a tractor emergency.
18. What are the two jobs of the emergency air line?
Supplying the trailer air tanks and controlling the trailer emergency brakes. The emergency line, also called the supply line, is the trailer's lifeline. First, it supplies air to the trailer air tanks so the trailer has its own stored pressure. Second, it controls the emergency brakes: as long as pressure holds, the brakes stay off, and any loss, from a breakaway tearing the hose or a broken fitting, brings the trailer emergency brakes on. That pressure loss also trips the tractor protection valve, sealing the tractor's air in. Red coding on this line keeps it from being confused with the blue service line.
19. What can happen if you cross the air lines on an older trailer without spring brakes?
You can drive away, but you will have no trailer brakes. Older trailers without spring brakes hide a crossed-line mistake instead of announcing it. With no spring brakes to hold the wheels, nothing stops you from driving away, yet the scrambled lines leave you with no working trailer brakes, a very dangerous state that may only reveal itself at the first real stop. That is why the handbook insists you always test the trailer brakes before driving: use the hand valve, or pull gently against the trailer with the supply control out, and confirm the brakes actually hold.
20. Which trailers and converter dollies are required to have ABS?
All trailers built on or after March 1, 1998. The ABS requirement is written around a build date: all trailers and converter dollies built on or after March 1, 1998, are required to have ABS. Many older units were fitted voluntarily, so the absence of a requirement does not prove a trailer lacks the system. On a pre-requirement trailer that gives no other clue, look under the vehicle for the ECU and the wheel speed sensor wires coming from the back of the brakes. Dollies built on or after that date must also carry their ABS lamp on the left side.
How this test works
Start a quick test, drill one topic, or run the whole bank. After each answer you see why it is right, why the choice you picked is wrong, and the handbook page to reread. Questions you miss come back in the retry round until you get them right. Back to the California DMV guide.
