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5.2 – Dual Air Brake System

Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one). The first system is called the “primary” system. The other is called the “secondary” system. (See Figure 5.4.)

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge). Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when the air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi.

The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brakes system fixed.

Multiple-Choice Questions:

Question #269 (1 of 5)

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The warning light and buzzer should shut off when the air pressure in both air brake systems rises to a value set by the manufacturer. This value must be:

  • Greater than 100 psi
  • Greater than 120 psi
  • Greater than 60 psi
  • Less than 60 psi
The warning light and buzzer should shut off when the air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi.
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Question #271 (2 of 5)

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What should you do if the air brake warning light and buzzer comes on while you're driving?

  • Stop right away and safely park the vehicle
  • Keep driving and watch the gauge. Do not lot it drop below 30 psi. If it does, park the vehicle.
  • Continue driving normally, but use the trailer hand control valve to stop the vehicle.
  • Push in the parking control knob immediately to preserve the air you have left in the system
The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle.
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Question #268 (3 of 5)

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Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of:

  • None of these are correct
  • 100 psi pressure in both the primary and secondary systems.
  • 100 psi in the primary system, 60 psi in the secondary system
  • 60 psi pressure in both the primary and secondary systems.
Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems.
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Question #267 (4 of 5)

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Which of the following describes a dual air brake system?

  • A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc
  • All these are correct
  • One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one).
  • The first system is called the “primary” system. The other is called the “secondary” system.
Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one). The first system is called the “primary” system. The other is called the “secondary” system. (See Figure 5.4.)
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Question #270 (5 of 5)

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In a dual air brake system, the warning light and buzzer should come on before the air pressure drops below what level in either system?

  • 90 psi
  • 60 psi
  • 120 psi
  • 30 psi
The warning light and buzzer should come on before the air pressure drops below 60 psi in either system.
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