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Brake fade, downhill braking, low air and parking

Air Brakes4 minManual section 5.4.5–5.4.8

Brakes turn speed into heat, and too much heat makes them fade. This lesson explains why brakes fade, the correct braking technique on long downgrades, what to do when the low-air warning comes on, and the rules for using parking brakes.

Brake fade or failure

Brakes slow the vehicle by making shoes or pads rub against drums or discs. This makes heat. Brakes are built to take a lot of heat, but they can fade or fail when they get too hot, which happens when you lean on them too much instead of using the engine's braking effect.

Brake fade happens in two ways. Too much heat causes chemical changes in the brake lining that reduce friction. Heat also makes the brake drums expand. As the drums expand, the shoes and linings must move farther to touch them, and the force of the contact is reduced. If you keep overusing the brakes, fade can grow until the vehicle cannot be slowed or stopped.

Adjustment also affects fade. Each brake has to carry its part of the load. A brake that is out of adjustment quits doing its part first. Then the other brakes overheat and fade, and there is not enough braking to control the vehicle. Brakes can go out of adjustment quickly, especially when they are hot, so check brake adjustment often.

Proper braking technique on downgrades

Remember: on a long and/or steep downgrade, the brakes are only a supplement to the braking effect of the engine. First get into the proper low gear. Then use this technique (often called snub braking):

  • Step 1: apply the brakes just hard enough to feel a definite slowdown.
  • Step 2: once you have slowed to about 5 mph under your "safe" speed, let off the brakes. Each application should take about 3 seconds.
  • Step 3: when your speed has risen back to your safe speed, repeat steps 1 and 2.

Example: your safe speed is 40 mph. Do not brake until you reach 40 mph. Then brake hard enough to slow gradually to 35 mph, and release. Repeat as often as needed until the bottom of the grade.

Low air pressure while driving

If the low air warning comes on, pull over and park safely as soon as you can. There might be an air leak. You can only brake in a controlled way while the tanks still hold enough air.

The spring brakes come on when pressure drops into the 20 to 45 psi range. A heavily loaded vehicle then needs a long distance to stop, because spring brakes are not on every axle. A lightly loaded vehicle, or one on a slippery road, may skid out of control when the spring brakes grab. It is far safer to stop while there is still air for the foot brake.

Parking brakes

Any time you park, use the parking brakes, except in the cases below. Pull the control knob out to apply, push it in to release. On newer vehicles the control is a yellow, diamond-shaped knob labeled "parking brakes." On older vehicles it may be a round blue knob or another shape, including a lever that swings side to side or up and down.

Do not use the parking brakes if:

  • the brakes are very hot (for example, you just came down a steep grade) — the heat can damage them; or
  • the brakes are very wet in freezing temperatures — they can freeze so the vehicle cannot move.

In those cases, hold the vehicle with wheel chocks on level ground. Wait for hot brakes to cool before setting the parking brakes. If the brakes are wet, brake lightly while driving in a low gear to warm and dry them.

If your vehicle does not have automatic air tank drains, drain the tanks when each work day ends to get rid of water and oil. If you skip this, the brakes could fail.

Do not walk away from your vehicle unless the parking brakes are set or the wheels are chocked — not even for a short time. The vehicle might roll away and cause injury and damage.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 5.4.5–5.4.8

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

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Review questions with answers

  1. What causes brake fade?
    1. Cold brake drums
    2. Excessive heat from overusing the service brakes instead of relying on engine braking
    3. Too much air pressure
    4. Driving in a low gear
    Show answer
    Excessive heat from overusing the service brakes instead of relying on engine braking

    Overusing the brakes creates too much heat; not relying on engine braking makes it worse.

  2. How does excessive heat cause brake fade?
    1. It shrinks the brake drums so shoes cannot reach
    2. It raises the air pressure
    3. It makes the linings grip harder
    4. It causes chemical changes in the lining that reduce friction, and makes the drums expand
    Show answer
    It causes chemical changes in the lining that reduce friction, and makes the drums expand

    Heat changes the lining and reduces friction; expanded drums mean shoes travel farther and press with less force.

  3. Why do brakes out of adjustment lead to brake fade?
    1. They stop doing their share, so the other brakes overheat and fade
    2. They create extra friction and stop the truck faster
    3. They make the compressor fail
    4. They only affect the parking brake
    Show answer
    They stop doing their share, so the other brakes overheat and fade

    Every brake must do its share. Out-of-adjustment brakes quit first and the rest overheat and fade.

  4. When can brakes get out of adjustment quickly?
    1. When they are cold
    2. When the truck is empty
    3. When they are hot
    4. Only after 100,000 miles
    Show answer
    When they are hot

    Brakes can go out of adjustment quickly, especially when hot, so check adjustment often.

  5. True or False: On a long, steep downgrade, the service brakes only add to the slowing effect of the engine; the engine does the main work.
    1. True
    2. False
    Show answer
    True

    True. Get into the proper low gear first; the brakes only add to engine braking.

  6. Once you are in the proper low gear on a downgrade, how hard should you apply the brakes?
    1. As hard as possible
    2. Just enough to hold the speed steady
    3. Barely touching the pedal the whole way
    4. Just hard enough to feel a definite slowdown
    Show answer
    Just hard enough to feel a definite slowdown

    Step 1 of the technique: brake just hard enough to feel a definite slowdown.

  7. Using the proper downhill braking technique, when should you release the brakes?
    1. When you reach your safe speed
    2. When your speed is about 5 mph below your safe speed
    3. When your speed is 15 mph below your safe speed
    4. Only at the bottom of the hill
    Show answer
    When your speed is about 5 mph below your safe speed

    Release when you are about 5 mph below your safe speed. Each application lasts about 3 seconds.

  8. In the proper downhill braking technique, each brake application should last about:
    1. 1 second
    2. 10 seconds
    3. 3 seconds
    4. 30 seconds
    Show answer
    3 seconds

    The application to drop about 5 mph should last about three seconds.

  9. Your safe speed on a downgrade is 40 mph. Using the proper technique, you should:
    1. Wait until 40 mph, brake to about 35 mph, release, and repeat
    2. Brake at 35 mph down to 30 mph
    3. Hold the brakes lightly to stay at 40 mph
    4. Brake at 45 mph down to 40 mph
    Show answer
    Wait until 40 mph, brake to about 35 mph, release, and repeat

    Do not brake until you reach 40 mph, then slow to 35 mph and release; repeat to the bottom.

  10. If the low air pressure warning comes on while driving, you should:
    1. Continue to the next terminal
    2. Stop and safely park as soon as possible
    3. Turn off the engine immediately in traffic
    4. Pump the brakes to build pressure
    Show answer
    Stop and safely park as soon as possible

    There may be a leak. Controlled braking only works while enough air remains, so stop and park safely right away.

  11. Why will a heavily loaded vehicle take a long distance to stop when the spring brakes come on?
    1. The ABS turns off
    2. The engine brake stops working
    3. The tires lose air
    4. The spring brakes do not work on all axles
    Show answer
    The spring brakes do not work on all axles

    Spring brakes do not work on all axles, so a heavy vehicle needs a long distance to stop.

  12. What can happen to a lightly loaded vehicle, or one on a slippery road, when the spring brakes come on?
    1. It stops in a very short distance with no risk
    2. Nothing; spring brakes are gentle
    3. It may skid out of control
    4. The compressor refills the tanks at once
    Show answer
    It may skid out of control

    With little weight, or on a slippery road, the vehicle may skid out of control when the spring brakes grab. Stop while you still have air.

  13. On older vehicles, the parking brake control may be:
    1. A round blue knob or a lever
    2. A red triangle button
    3. A pedal on the right side
    4. A switch on the steering column only
    Show answer
    A round blue knob or a lever

    Older vehicles may use a round blue knob or another shape, including a lever.

  14. You just came down a long, steep grade and your brakes are very hot. How should you park?
    1. Set the parking brakes right away
    2. Use wheel chocks on a level surface and let the brakes cool before using the parking brakes
    3. Pump the service brakes while parked
    4. Leave the engine running in gear
    Show answer
    Use wheel chocks on a level surface and let the brakes cool before using the parking brakes

    Very hot brakes can be damaged by the parking brakes. Chock the wheels and let them cool first.

  15. Why should you avoid using the parking brakes when the brakes are very wet in freezing temperatures?
    1. They will overheat
    2. They will drain the air tanks
    3. They will turn on the ABS lamp
    4. They can freeze so the vehicle cannot move
    Show answer
    They can freeze so the vehicle cannot move

    Wet brakes can freeze in place. Chock the wheels instead; to dry wet brakes, use them lightly while driving in a low gear.

  16. If your brakes are wet, how can you dry them?
    1. Drain the air tanks
    2. Park in the sun with the parking brake on
    3. Use the brakes lightly while driving in a low gear
    4. Drive fast without braking
    Show answer
    Use the brakes lightly while driving in a low gear

    Light brake use in a low gear heats and dries wet brakes.

  17. True or False: If you will be away from your truck for just a few minutes, you can skip setting the parking brake.
    1. True
    2. False
    Show answer
    False

    False. Never leave a vehicle unattended without applying the parking brakes or chocking the wheels; it could roll away.

  18. How often should you drain air tanks that do not have automatic drains?
    1. At the end of each working day
    2. Once a week
    3. Every 1,000 miles
    4. Only when water is visible
    Show answer
    At the end of each working day

    Without automatic drains, drain the tanks when each work day ends to get rid of water and oil.

  19. To apply the parking brakes on a newer air brake vehicle, you:
    1. Push the yellow knob in
    2. Press the brake pedal hard
    3. Turn the knob a quarter turn
    4. Pull the yellow diamond-shaped knob out
    Show answer
    Pull the yellow diamond-shaped knob out

    Pull the yellow diamond-shaped "parking brakes" knob out to apply; push it in to release.