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5.4 Using Air Brakes (continued)

Brake Fading or Failure

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Brake fade also is affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

Proper Braking Technique

Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • Apply the brakes just hard enough to feel a definite slowdown.
  • When your speed has been reduced to approximately 5 mph below your “safe” speed, release the brakes. (This brake application should last about 3 seconds.)
  • When your speed has increased to your “safe” speed, repeat the first two steps.

For example, if your “safe” speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.

Low Air Pressure

If the low air pressure warning light comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops into the 20 to 45 psi range. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on. It is much safer to stop while there is enough air in the tanks to use the foot brake.

Parking Brakes

Any time you park, use the parking brakes, except as noted. Pull the parking brake control knob out to apply the parking brakes. Push it in to release them. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles. On older vehicles, it may be a round blue knob or some other shape, including a lever that swings from side to side or up and down.

Do not use the parking brakes if the brakes are very hot (i.e., from just coming down a steep grade), or if the brakes are very wet in freezing temperatures. If the brakes are used when very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks to hold the vehicle. Let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.

Test Your Knowledge

  • Why should you be in the proper gear before starting down a hill?
  • What factors can cause brakes to fade or fail?
  • The use of brakes on a long, steep downgrade is only a supplement to the braking effect of the engine. True or False?
  • If you are away from your vehicle only a short time, you do not need to use the parking brake. True or False?
  • How often should you drain air tanks?

Study section 5.4 if you can't answer all of these questions.

We've seen "Brake Fade" discussed a few times now and in the next few paragraphs we will get into a bit more detail. There's nothing too specific to memorize, but you should have a good overall understanding of what Brake Fade is and the consequences of Brake Fade.
In the below bullet points and example, pay particular attention to what a "safe speed" is and how it is used on steep downgrades. Also remember that your service brakes should only be used as a supplement to engine braking.

Spring brakes will automatically activate when air pressure drops between 20 and 45 psi. You should already have this memorized and there is a very good chance it will show up on the written exam.

Side note: Do you remember when the low air pressure warning device will activate? If you said before 60 psi, you're correct! As soon as your low air warning device activates, pull off the roadway and park as soon as you can safely do so, before pressure drops further and activates your spring brakes.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

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

Spring brakes should activate when air pressure drops to:
  • Between 20 and 45 psi
  • Between 10 and 20 psi
  • Between 50 and 75 psi
  • Between 100 and 120 psi

Quote From The CDL Manual:

The spring brakes will come on when the air pressure drops into the 20 to 45 psi range

Next
What is the proper braking technique on long or steep downgrades?
  • Apply the brakes just hard enough to feel a definite slowdown, when your speed has been reduced to approximately 5mph below your "safe" speed, release the brakes and repeat as necessary
  • Fluctuate between "hard braking" and "soft braking" but never fully release the brakes during a downgrade
  • Apply the brakes enough to keep a constant speed during the entire downgrade
  • Begin the downgrade at a slow enough speed that brakes will not be necessary during the entire grade

Quote From The CDL Manual:

Proper Braking Technique - Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • 1. Apply the brakes just hard enough to feel a definite slowdown.
  • 2. When your speed has been reduced to approximately 5 mph below your "safe" speed, release the brakes. (This brake application should last about 3 seconds.)
  • 3. When your speed has increased to your "safe" speed, repeat steps 1 and 2.
Prev
Next
Why does brake fade occur?
  • Brake fade occurs when brakes become too cold during freezing weather conditions
  • Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect
  • Brake fade occurs when using the engine brake for too long on a steep downgrades
  • Brake fade occurs when excessive moisture builds onto the brake linings

Quote From The CDL Manual:

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums.

As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Brake fade also is affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

TruckingTruth's Advice:

Brake fade is the #1 cause of all brake failures during mountain driving. Most trucks are equipped with engine braking devices which should be used as often as possible.

Prev
Next
How can Brake Fade be avoided?
  • Properly calibrating slack adjusters
  • All of these are ways to help avoid Brake Fade
  • Avoiding excessive use of the service brakes
  • Making sure all brake linings are in good condition

Quote From The CDL Manual:

Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Prev
Next
What is a "safe speed"?
  • Speeds of 55mph or below are considered safe speeds for maximum fuel mileage
  • 10mph under the posted speed limit is considered a safe speed
  • A predetermined speed for descending down a long or steep grade, which helps determine when brakes should be applied or released
  • The yellow signs before curves and ramps are designated safe speeds

Quote From The CDL Manual:

Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • 1. Apply the brakes just hard enough to feel a definite slowdown.
  • 2. When your speed has been reduced to approximately 5 mph below your "safe" speed, release the brakes. (This brake application should last about 3 seconds.)
  • 3. When your speed has increased to your "safe" speed, repeat steps 1 and 2.
Prev
Next
When should air tanks be drained?
  • Every month
  • At least once per week
  • At the end of each day
  • None of these answers are correct

Quote From The CDL Manual:

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

Prev
Next
Which of the following statements about Brake Fade is incorrect?
  • As overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced
  • Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums
  • Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all
  • While brake fade was once a very common problem, it is very rare with modern brake cooling mechanisms

Quote From The CDL Manual:

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.


Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Brake fade also is affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

TruckingTruth's Advice:

While engine brakes are a fantastic tool in helping to prevent brake fade, the number one cause of brake failure in hilly conditions is brake fade.

Prev
Next
The following are all true about parking brakes, except:
  • When there are freezing temperatures and the brakes are very wet, the brake linings may freeze to the brake drums
  • When the brakes are very hot (i.e. just after coming down a steep grade) wheel chocks should be used instead of the parking brake
  • Older vehicles may have a round blue knob for the parking brake in place of the yellow diamond shaped knob currently used in modern equipment
  • All of these answers are true

Quote From The CDL Manual:

Any time you park, use the parking brakes, except as noted. Pull the parking brake control knob out to apply the parking brakes. Push it in to release them. The control will be a yellow, diamond-shaped knob labeled "parking brakes" on newer vehicles. On older vehicles, it may be a round blue knob or some other shape, including a lever that swings from side to side or up and down.

Do not use the parking brakes if the brakes are very hot (i.e., from just coming down a steep grade), or if the brakes are very wet in freezing temperatures. If the brakes are used when very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks to hold the vehicle. Let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.


Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage

Prev
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Using Air Brakes And Stopping Distance
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Driving Combination Vehicles

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