Air Brakes Test Study Guide for the CDL

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Low Pressure Warning System

The Low Pressure Warning System is designed to warn a driver of a dangerous low air pressure condition. If the pressure of the brake system drops, a warning light is illuminated and a buzzer sounds. If the pressure of the brake system continues to drop, the warning system will lock up the brakes. (More accurately, the parking brake valve and trailer protection valve will pop out; causing the spring brakes to lock-up.)

Parts of the Low Pressure Warning System

The low pressure warning light illuminates above 60 psi. The parking brake valve and trailer protection valve pop out between 20 psi and 45 psi. Older systems were designed to pop out the parking brake valve first, then the trailer protection valve—but this has become uncommon. The parking brake should not pop out first.

Other pieces of equipment that are not part of the warning system (but used during tests and inspections) are:

  • pressure gauge(s) that monitor air pressure in various tanks

  • the parking valve (yellow knob)

  • trailer protection valve (red valve)

  • air compressor

  • governor (controls the air compressor and maintains air pressure in the primary tank between 100 psi and 125 psi)

Trucks will usually have two pressure tanks and two pressure gauges (primary and secondary). Older straight trucks might have only one tank and one gauge (primary only).

When a truck is first started, it is normal for the air pressure to be low. Warning lights will be illuminated and buzzers will sound. Also, it might be impossible to release the truck’s brakes until the air pressure builds up.

Warning: Never operate the truck with less than 100 psi in all air tanks.

Some older equipment might have a Wig-Wag instead of a warning light and buzzer. A Wig-Wag is a brightly colored paddle that swings down, in full view of the driver, when triggered by the warning system.

Air Brake Check

To pass your CDL driving test, you will be required to perform an Air Brake Check. An Air Brake Check is a group of individual tests that collectively determine if a truck’s air brakes are safe to operate.

An Air Brake Check is also a handy way to diagnose problems a driver might be having with the brakes on a truck. Any time a driver is assigned to a new or unfamiliar piece of equipment, an Air Brake Check should be performed.

The Air Brake Check is broken down into two distinct groups of tests: Pump Down tests and Pump Up tests.

Note 1: Always perform an Air Brake Check on a flat, level surface. Always chock the wheels of your truck before performing an Air Brake Check.

Note 2: Some DMV testers want the truck in neutral during all of these tests. Other testers want the truck in gear with the clutch engaged (out) during the Pump Down tests, and the truck taken out of gear prior to starting the truck for the Pump Up tests. Ask the tester if you are uncertain.

Note 3: Some DMV testers will test air leakage before the Pump Down tests, instead of at the very end of the tests. The DMV tester will instruct you prior to the test.

Important: Never attempt to start the truck in gear with the clutch engaged (out). This will cause the truck to move. In a testing situation, this is an automatic “fail.”

Hint 1: You are going to need a watch with a second hand to complete these tests. (Or, some other way of timing these tests.)

Hint 2: There are a lot of steps to these tests. These steps need to be memorized. Practice beforehand. Practice. Practice. Practice. Remain calm. The DMV tester wants you to do well, and pass these tests. The tester won’t do anything to confuse you or “trick” you.

Pump Down Test

  • Test Low-Pressure Warning Signal
  • Parking Brake Valve & Trailer Protection Valve

With air tanks charged with enough air that the low-pressure warning light is not on:

  1. Check that wheels are chocked.

  2. Switch the engine off.

  3. Switch the ignition key to “on” (so that the dashboard lights are illuminated).

  4. Pump the brake pedal repeatedly, until the low pressure warning light illuminates and the buzzer sounds. The light and buzzer should activate before the tank pressure is below 60 psi (on dual systems: the tank with the lowest pressure).

  5. Switch the ignition key to “off” so that the buzzer will stop sounding. The low pressure warning light will extinguish at the same time.

  6. Resume pumping the brake pedal until the parking brake valve and trailer protection valve pop out. (Straight trucks do not have a trailer protection valve.) This should be between 20 psi and 45 psi.

Pump Up Test

Air Pressure Buildup

  1. Double-check wheels are chocked.

  2. Double-check that the transmission is in neutral.

  3. Start the engine.

  4. Note the pressure on the primary air gauge. Pressure should rise from 85 psi to 100 psi within 45 seconds at normal RPMs.

  5. Observe when the pressure stops increasing in the primary tank. This should be approximately 125 psi.

  6. With the engine running, pump the brake pedal repeatedly until the governor starts the compressor and the pressure in the primary tank starts increasing. This should be approximately 100 psi.

Note: If the truck has oversized air tanks, it could take longer than 45 seconds for the pressure to rise from 85 psi to 100 psi (consult the manufacturer’s specifications). Single tank systems built before 1975 are tested differently.

Air Leakage Rate

  1. Double-check that wheels are chocked.

  2. With the air tanks fully charged (approximately 125 psi), switch off the engine.

  3. Release (press in) the parking valve (yellow knob) and trailer protection valve (red valve).

  4. Allow the air system to stabilize.

  5. Fully press the brake pedal.

  6. When the pressure of the primary tank stabilizes, watch the pressure on the primary gauge.

  7. The pressure should drop no more than 3 psi per minute for a straight truck or 4 psi per minute for a combination vehicle.


  • Find a comfortable position for your foot on the brake pedal prior to starting this test. If you jiggle your foot or shift your leg during the test, you could move the pedal enough to cause the test to fail. A minute is a long time to hold a heavy brake pedal steady.

  • Memorize the numbers in these tests. You will see them during the CDL written tests:

    • The low-pressure warning light illuminates above 60 psi.

    • The parking brake valve and trailer protection valve pop out between 20 psi and 45 psi.

    • There is a 3 psi per minute loss for a straight truck.

    • There is a 4 psi per minute loss for a combination vehicle.

    • The governor controls the compressor and maintains pressure in the primary tank between 100 psi and 125 psi.

More hints:

  • The parking valve is the diamond-shaped knob (yellow).

  • Trailer protection valve is the octagon-shaped knob (red).

  • When testing on a dual system, always use the primary air pressure gauge.

  • The low-pressure warning signal (light and buzzer) uses a pressure switch on the wet tank.

ABS (Anti-lock Braking System)

Anti-lock Braking Systems use wheel speed sensors and ECUs (Electronic Control Units) to prevent the wheels on a truck locking up. An ABS doesn’t necessarily decrease braking distance. It is designed to allow better control of a truck during hard braking. When the ABS is not needed, the brakes operate normally.

If your tractor has ABS, there will be a yellow ABS warning light on the dashboard. This warning light will illuminate briefly when the tractor is started. On older tractors, this warning light will be illuminated until the tractor reaches 5 miles per hour.

If your trailer has ABS, there will be an extra, yellow “marker style” malfunction light on the left side of the trailer. This malfunction light will be on the front of the trailer or the rear corner. You should be able to see this light in the tractor’s left side mirror. Sometimes, the malfunction light is labeled “ABS.” Sometimes, the malfunction light is an unlabeled yellow light next to the red rear marker light. When the vehicle is switched on, the light will flash briefly then turn off (which is a bulb check). On older trailers, the light will remain on until the trailer gets above 5 miles per hour. If the bulb remains on, you may have a fault. If the bulb flashes in a repeating pattern, write down the pattern of flashes. A mechanic can compare the pattern of flashes with a lookup chart to help diagnose any problems.

  • ABS provides for better control on slippery surfaces. It is not designed to stop a truck faster.

  • ABS does not allow you drive faster, or follow closer. If your ABS is not working properly, the air brakes will still operate normally.

  • ABS does not increase braking power or decrease stopping distance.

Other Considerations

There are a few special considerations you should know about air brakes: the stopping distance, parking, and low-pressure warnings.

  • The stopping distance for a vehicle with air brakes is slightly longer than that of a normal vehicle because of the added delay needed for air to flow through the air lines to the brake cylinders. Because of this, there are circumstances that may require extra care when braking.

  • Braking on downgrades can cause excessive heat in the brakes, causing them to fade. On downgrades, it is best to use a lower gear and to drive slow enough that on-and-off-braking will keep you from increasing speed. On-and-off-braking alone has been proven ineffective and should not be used as an alternative to downshifting.

  • Do not use a light, steady pressure on the brakes on a downgrade. This will cause excess heat in the braking system. Use on-and-off-braking.

  • If you ever get a low-pressure warning as you drive, pull over and park as soon as it is safe to do so. A low-pressure warning may indicate a leak in the air brake system. You will only be able to do controlled braking if you have sufficient pressure in the air tanks.

  • Finally, to safely park your vehicle, the parking brakes must be engaged. The only exceptions are when the brakes are very hot or if the brakes are very wet in freezing conditions. In these cases, it is best to use wheel chocks to hold the vehicle.

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