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Parts Of An Air-Brake System: Part 5

5.1.14 – Spring Brakes

All trucks, truck tractors and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force (because air pressure can eventually leak away). Spring brakes are usually used to meet these needs.

When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This allows the springs to apply the brakes. A leak in the air brake system, which causes all the air to be lost, will apply the spring brakes.

Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi. Do not wait for the brakes to come on automatically. When the low air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away while you can still control the brakes.

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.

Multiple-Choice Questions:

Question #260 (1 of 6)

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Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of:

  • 80 to 110 psi
  • 120 to 150 psi
  • 20 to 45 psi
  • 60 to 90 psi
Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi. Do not wait for the brakes to come on automatically. When the low air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away while you can still control the brakes.
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Question #256 (2 of 6)

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Why is spring force used instead of air for emergency and parking brakes?

  • Air pressure can eventually leak away
  • Springs produce more power than air
  • All of these are correct
  • Springs prevent movement in both directions, air may not

All trucks, truck tractors and buses must be equipped with emergency brakes and parking brakes. Mechanical force must hold them on because air pressure can eventually leak away. Spring brakes are usually used to meet these needs.

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Question #261 (3 of 6)

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The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly:

  • Neither the regular brakes nor the emergency/parking brakes will work right.
  • Brake adjustment doesn't affect the spring brakes or the emergency/parking brakes
  • The regular brakes will not work but the emergency/parking brakes will
  • The regular brakes will work but the emergency/parking brakes will not
The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
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Question #259 (4 of 6)

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If there is a leak in the air brake system and it loses all of its air, what happens?

  • It will apply the hydraulic brakes
  • It will apply the spring brakes
  • The trailer brakes will apply, but the tractor brakes will not
  • You will lose your brakes completely
A leak in the air brake system, which causes all the air to be lost, will apply the spring brakes.
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Question #258 (5 of 6)

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When you want to apply the parking brakes, what does the parking brake control in the cab actually do?

  • None of these are correct
  • It forces air into both sides of the brake chamber at once and applies the brakes
  • It forces the S-Cam to lock in place, preventing the application of the brakes
  • It allows the driver to let the air out of the spring brakes. This allows the springs to apply the brakes.
A parking brake control in the cab allows the driver to let the air out of the spring brakes. This allows the springs to apply the brakes.
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Question #257 (6 of 6)

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Which of the following is true about air brakes?

  • When driving, powerful spring brakes are engaged by air pressure. If the air pressure is removed, the springs release the brakes.
  • The spring pressure and air pressure combine to release the forces on the brakes, allowing the truck to roll
  • None of these are correct
  • When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes.
When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes.
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