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Study guide

CDL air brakes: the pressures and the brake check

Learn at what pressure each part acts, how much air a truck may lose in a minute, and the check that uses both.

Who takes it
Anyone who will drive a vehicle with air brakes, air over hydraulic included.
To pass
80 percent.
Questions
25 in Michigan and Pennsylvania. Check your state's manual.
If you fail
Your permit or license is restricted: no vehicle with air brakes.
On this page
  1. The pressures, from the top down
  2. Three more figures to know
  3. The seven parts of the final air brake check
  4. Worked example: three gauge readings
  5. Five common mistakes
  6. Three practice questions
  7. Sources

The pressures, from the top down

Learn these five in gauge order, top to bottom, and you always know where on the gauge you are.

150 psi
The safety valve is usually set to open here. If it ever releases air, something is wrong: have a mechanic fix it. Manual 5.1
125 psi
Cut-out, about: the tanks are full and the governor stops the compressor. Manual 5.1
100 psi
Cut-in, about: the governor lets the compressor pump again. It is also the least a dual air system needs in both halves before you drive. Manual 5.1, 5.2
55 psi
The low air pressure warning must come on before the tanks fall below this (on older vehicles, below half the governor's cut-out pressure). Manual 5.1
20 to 45 psi
The spring brakes come fully on. On a tractor-trailer the tractor protection valve closes in the same range. Manual 5.1, 5.3

Three more figures to know

32 feet
What brake lag adds at 55 mph on dry pavement: the air needs half a second or more to reach the brakes. Total stopping distance at 55 mph is then over 450 feet. Manual 5.4
1 inch
On s-cam brakes: park on level ground, chock the wheels, then release the parking brakes and pull hard on each slack adjuster you can reach. If it moves more than about an inch where the push rod attaches, it probably needs adjustment. Manual 5.3
5 mph
On a long downgrade, once you are in the proper low gear, brake until you are about 5 mph under your safe speed, then release. Repeat when you are back at the safe speed. Manual 5.4

The seven parts of the final air brake check

The manual gives them in this order. Each one checks a figure from the list above. The pressures are the typical ones: where the manufacturer specifies others, those apply. Manual 5.3

  1. Governor

    At a fast idle the compressor should stop pumping at about 125 psi. Step on and off the brake: it should start again at about 100.

  2. Air leakage

    With the system fully charged (typically 125 psi), turn the engine off and release the parking brake. With the pedal up, the loss in one minute must stay under 2 psi for a single vehicle and under 3 for a combination. Then hold the pedal down for a minute, wheels chocked if necessary: more than 3 psi (single) or 4 (combination) is too much.

  3. Low pressure warning

    Key on, engine off. Step on and off the pedal. The warning must come on before the pressure drops below 55 psi.

  4. Spring brakes

    Keep stepping on and off the pedal. The parking brake valve should pop out at 20 to 45 psi, and on a tractor-trailer the tractor protection valve with it.

  5. Build-up

    With the engine at operating rpm, a dual air system should climb from 85 to 100 psi within 45 seconds. That figure is for minimum-size air tanks: with larger tanks it can take longer and still be safe, so check the manufacturer's specifications.

  6. Parking brake

    Stop, set it, and pull gently against it in a low gear. It must hold.

  7. Service brakes

    Wait for normal air pressure, release the parking brake, roll forward at about 5 mph and brake firmly. Pulling to one side, an odd feel or a delay is a fault.

Worked example: three gauge readings

The leakage limits turn on two words: less than, or more than. Set each reading against its limit.

  • Straight truck, pedal up. The gauge falls from 125 to 122 psi in one minute.

    Too much. A single vehicle must lose less than 2 psi with the brakes released. It lost 3.

  • Same truck, pedal held down. 3 psi lost in one minute.

    Within the limit. With the foot brake applied, the loss is too much only when it is more than 3 psi for a single vehicle.

  • Tractor-trailer, pedal up. 3 psi lost in one minute.

    Too much. A combination must lose less than 3 psi with the brakes released. Exactly 3 is not less than 3.

Five common mistakes

  • Common answerCut-in is the higher number.

    The manual 5.1Cut-out is the top, about 125 psi: the compressor is cut out of the job. Cut-in is the bottom, about 100: it is brought back in.

  • Common answerABS lets me stop in a shorter distance.

    The manual 5.1ABS keeps the wheels from locking so that you keep control. It does not necessarily shorten your stopping distance. Brake as you always have.

  • Common answerAn automatic slack adjuster that is out of adjustment just needs adjusting.

    The manual 5.3It is the symptom of a fault. Take the vehicle to a repair facility. Adjusting it by hand is a temporary measure for an emergency.

  • Common answerWhen the low air warning comes on, I can drive until the spring brakes take over.

    The manual 5.4Stop and park safely right away. The spring brakes come on at 20 to 45 psi, they do not act on all axles, and a light truck or one on a slippery road can skid when they do.

  • Common answerOn a long downgrade the brakes do the work.

    The manual 5.4The engine does, in a low gear. The brakes only add to it: brake to about 5 mph under your safe speed, release, repeat. Each application lasts about three seconds.

Three practice questions

Pick an answer. The reason and the manual's own sentence follow.

You shut off the engine of a tractor-trailer at full pressure and release the parking brake. With your foot off the pedal, the gauge falls 3 psi in one minute. What does that tell you?

Show the answer

The answer is B. With the brakes released, the limit is less than 2 psi in one minute for a single vehicle and less than 3 for a combination. Exactly 3 is not less than 3, so find and fix the leak before you drive.

Commercial driver manual, section 5.3The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles.

The low air pressure warning comes on while you are driving. What do you do?

Show the answer

The answer is B. There may be a leak, and you can brake under control only while enough air is left. If you keep going, the spring brakes come on at 20 to 45 psi, which can mean a very long stop or a skid.

Commercial driver manual, section 5.4If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible.

At 55 mph on dry pavement, about how much distance does air brake lag add to your stop?

Show the answer

The answer is B. After you press the pedal, the air needs half a second or more to reach the brakes. At 55 mph that is about 32 feet, on top of perception, reaction and braking distance.

Commercial driver manual, section 5.4The air brake lag distance at 55 mph on dry pavement adds about 32 feet.

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Sources

Reviewed by CDL Greenlight on October 13, 2026. CDL Greenlight writes and checks this page. Every figure is the one the commercial driver manual prints, with its section named beside it. Where your state has a rule of its own, your state's manual decides.

Found a mistake? Write to support@cdlgreenlight.com. Corrections we have published