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5.4 Using Air Brakes

Normal Stops

To apply the air brakes during normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is down close to idle. When stopped, select a starting gear.

Emergency Stops

If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop and you use the brakes correctly

You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the “controlled braking” method or the “stab braking” method.

Controlled braking:

With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

Stab braking:

Use only on vehicles without anti-lock systems.

  • Apply the brake all the way.
  • Release the brakes when the wheels lock up.
  • As soon as the wheels start rolling, put on the brakes fully again. It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.

Anti-lock Brakes

New trucks and truck-trailer vehicles are equipped with anti-lock brakes. The anti-lock braking system is different than the normal air-brake system but works on the same principle.

Vehicles that have anti-lock brakes have a yellow light near the driverʼs rear side of the vehicle with the letters ABS stenciled above the light. Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes. This lamp will remain constant if there is a malfunction in the anti-lock brake system.

For normal or emergency stopping using anti-lock brakes, the driverʼs foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system. On the air-brake system the driver must pump or use stab braking in an emergency. If the anti-lock brake system fails or malfunctions, the driver must resort to stopping the vehicle by using the normal air-brake method. If an emergency arises, the driver should use the controlled or stab braking method. The anti-lock brake system should be serviced as soon as possible.

Stopping Distance

Stopping distance was discussed in Section 2: Speed and Stopping Distance. With air brakes, there is an added delay – the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light /medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one-half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of four different factors:

Perception Distance
+ Reaction Distance
+ Brake Lag Distance
+ Effective Braking Distance
------------------------------
= Total Stopping Distance

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet. This is longer than a football field.

Make sure you are familiar with the differences between controlled braking and stab braking by reading the below explanations. Questions on the written exam frequently ask about these braking methods.
Remember: Only use the stab braking method on vehicles without Anti-Lock Brakes.
It's important for you to understand that there is a lag between the time you depress the brake pedal and when the brakes actually engage. For an air brake system, the delay can be 1/2 second or more.
It is highly likely that you will be tested on the below formula. Make sure you have the stopping distance formula below memorized.
Be sure to memorize the average total stopping distance at 55mph with good brakes and in good traction (more than 300 feet).

Review Questions - Click On The Picture To Begin...

For an average driver traveling 55mph under good traction and brake conditions, total stopping distance using air brakes will have a stopping distance of:
  • More than 250 feet
  • More than 400 feet
  • More than 300 feet
  • More than 550 feet

Quote From The CDL Manual:

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet. This is longer than a football field.

TruckingTruth's Advice:

Brake lag distance isn't only something you'll see in a CDL manual or training course. This is a real issue and one of the biggest downfalls to an air brake system.

Next
Should you use stab braking on a vehicle equipped with Anti-Lock Brakes?
  • No, stab braking should never be used if a vehicle is equipped with Anti-Lock Brakes
  • Yes, but only on dry surfaces
  • Yes, but only when the Anti-Lock Brake system fails and an emergency arises
  • Yes, stab braking should always be used in conjunction with Anti-Lock Brakes

Quote From The CDL Manual:

If the anti-lock brake system fails or malfunctions, the driver must resort to stopping the vehicle by using the normal air-brake method. If an emergency arises, the driver should use the controlled or stab braking method. The anti-lock brake system should be serviced as soon as possible.

Prev
Next
Which of the following accurately describes Stab Braking?
  • Apply the service brake all the way, release the brakes when the wheels lock up, then apply the brake again repeating the process as many times as necessary
  • Apply the brakes as hard as you can without locking the wheels
  • Control the pressure so the vehicle comes to a smooth, safe stop
  • Applying and releasing the emergency brake until you're able to come to a complete stop

Quote From The CDL Manual:

Stab braking: Use only on vehicles without anti-lock systems.

  • Apply the brake all the way.
  • Release the brakes when the wheels lock up.
  • As soon as the wheels start rolling, put on the brakes fully again. (It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.)

TruckingTruth's Advice:

It will be important for you to understand the difference in the following terms (make sure you memorize the meaning of each):

  • Normal Stops: To apply the air brakes during normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is down close to idle. When stopped, select a starting gear.
  • Emergency Stops: If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop and you use the brakes correctly.
  • Controlled Braking: With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

Stab Braking Use only on vehicles without anti-lock systems.

  • Apply the brake all the way.
  • Release the brakes when the wheels lock up.
  • As soon as teh wheels start rolling, put on the brakes fully again. (It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.)
Prev
Next
How do you properly use Anti-Lock Brakes?
  • The driver's foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system
  • Drivers should use Stab Braking in which they depress the brake, release when the wheels lock up, then depress the brake again repeating as many times as necessary
  • None of these answers are correct
  • Anti-Lock Brakes are only used when there is a significant amount of brake fading

Quote From The CDL Manual:

For normal or emergency stopping using anti-lock brakes, the driver's foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system. On the air-brake system the driver must pump or use stab braking in an emergency. If the anti-lock brake system fails or malfunctions, the driver must resort to stopping the vehicle by using the normal air-brake method. If an emergency arises, the driver should use the controlled or stab braking method. The anti-lock brake system should be serviced as soon as possible.

Prev
Next
What is Controlled Braking?
  • Used during normal stops, control the pressure so the vehicle comes to a smooth, safe stop
  • Apply the brakes as firmly as possible, then when the wheels lock up, release the brakes and repeat the process as many times as needed
  • Apply the brakes as hard as possible and lock up the wheels
  • Apply the brakes as hard as you can without locking the wheels

Quote From The CDL Manual:

Controlled braking: With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

TruckingTruth's Advice:

It will be important for you to understand the difference in the following terms (make sure you memorize the meaning of each):

  • Normal Stops: To apply the air brakes during normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is down close to idle. When stopped, select a starting gear.
  • Emergency Stops: If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop and you use the brakes correctly.
  • Controlled Braking: With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

Stab Braking Use only on vehicles without anti-lock systems.

  • Apply the brake all the way.
  • Release the brakes when the wheels lock up.
  • As soon as teh wheels start rolling, put on the brakes fully again. (It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.)
Prev
Next
How can you tell a trailer is equipped with Anti-Lock Brakes?
  • There will be a bright red flag attached to the airlines underneath the trailer with the words "Anti-Lock" on the flag
  • The words "anti-lock brake system installed" will be located on the front of the trailer
  • There will be a yellow light near the driver's rear side of the trailer with the letters "ABS" stenciled above the light
  • All trucks are required to have an indicator light on the dash so drivers know if a trailer has Anti-Lock Brakes

Quote From The CDL Manual:

Vehicles that have anti-lock brakes have a yellow light near the driver's rear side of the vehicle with the letters ABS stenciled above the light.

Prev
Next
All of the following indicate a truck is equipped with an Anti-Lock Brake System, except:
  • Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes
  • If there is a malfunction in the ABS system, a yellow warning lamp will illuminate and stay on
  • Vehicles that have anti-lock brakes have a yellow light near the driver's rear side of the vehicle with the letters ABS stenciled above the light
  • All trucks and trailers are required to be equipped with Anti-Lock brake systems

Quote From The CDL Manual:

Vehicles that have anti-lock brakes have a yellow light near the driver's rear side of the vehicle with the letters ABS stenciled above the light. Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes. This lamp will remain constant if there is a malfunction in the anti-lock brake system.

Prev
Next
What is the air brake lag distance at 55 mph on dry pavement?
  • About 38 feet
  • About 47 feet
  • About 32 feet
  • About 25 feet

Quote From The CDL Manual:

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet. This is longer than a football field.

TruckingTruth's Advice:

Brake lag is something that all air brake vehicles have. It takes about one-half second for the brakes to start working after you've depressed the brake pedal. This is just one more reason why you should maintain a much larger following distance while driving a commercial vehicle.

Prev
Next
All of the following will increase your stopping distance, except:
  • Effective Braking Distance
  • Brake Lag Distance
  • Applied Friction Distance
  • Reaction Distance

Quote From The CDL Manual:

With air brakes, there is an added delay - the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one-half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of four different factors:

Perception Distance
+ Reaction Distance
+ Brake Lag Distance
+ Effective Braking Distance
---------------------------
= Total Stopping Distance

TruckingTruth's Advice:

Be certain you have the stopping distance formula memorized as it will almost definitely show up on your written exam.

Prev
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