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5.2 Dual Air Brake Systems

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that 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 one is called the “secondary” system.

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 air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi.

Test Your Knowledge

  • Why must air tanks be drained?
  • What is a supply pressure gauge used for?
  • What are spring brakes?
  • Front wheel brakes are good under all conditions. True or False?

Study section 5.1 if you can't answer all of these questions.

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 brake system fixed.

5.3 Inspecting Air Brake Systems

You should use the basic 7-Step Inspection Procedure described in Section 2 to inspect your vehicle. There are more items to inspect on a vehicle with air brakes than one without them. These items are discussed below, in the order that they fit into the 7-Step Method.

STEP 2: Engine Compartment Checks

Check air compressor drive belt if compressor is belt driven. If the air compressor is belt driven, check the condition and tightness of the belt. The belt should be in good condition.

STEP 5: Walkaround Inspection

Check manual slack adjusters on S-Cam brakes:

Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjusting. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

Check brake drums or discs, linings and hoses:

Brake drums or discs must not have cracks longer than one-half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.

Memorize the following:

  • One system operates the regular brakes on the rear axles and the other operates the regular brakes on the front axle and possibly one rear axle.
  • Both systems supply air to the trailer if there is one.
Remember: A dual air brake system has two separate air brake systems that use a single set of brake controls.

Make sure you know what the two separate systems are called:

  • The first system is called the primary system.
  • The other is called the secondary system.
Be sure to memorize this. You air system should be at a minimum of 100 psi before you begin driving (side note: A fully charged system is approximately 120 psi.)
We've seen this figure a couple times now. The low air warning device must activate before air pressure reaches 60 psi and may not turn off until air pressure rises above 60 psi. This is important and should be memorized.
And here it is again! The low air pressure warning device needs to activate before air pressure drops below 60 psi. This will continue to show up over and over again and very often shows up on the written exam!

Knowing how to check manual slack adjusters is very important. A few key points to remember:

  • Park on level ground and chock the wheels.
  • Turn off the parking brake.
  • Use gloves and pull on each slack adjuster by hand.
  • Slack adjusters should not move more than about 1-inch.

This entire paragraph is very important. Make sure you're familiar with everything stated below. You should pay particular attention to the following:

  • Linings should not have cracks longer than one-half the width of the friction area.
  • Linings are free of oil and grease.
  • Make sure air hoses are not cut or worn due to rubbing.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

OOS:

When a violation by either a driver or company is confirmed, an out-of-service order removes either the driver or the vehicle from the roadway until the violation is corrected.

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

Which of the following statements about Dual Air Brake Systems is false?
  • The warning light and buzzer should come on before the air pressure drops below 60 psi in either system
  • The first system is called the "parent" system an the other is called the "child" system
  • A dual air brake system has two separate air brake systems that use a single set of brake controls
  • Most newer heavy-duty vehicles use dual air brake systems

Quote From The CDL Manual:

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that 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.

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 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 brake system fixed.

TruckingTruth's Advice:

It is very likely the vehicle you drive will have a dual air brake system, so be sure to understand how the system works not only for your exams, but for when you are performing your daily job duties.

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In a Dual Air Brake System:
  • The low air warning should not activate until 30 psi since there are two air brake systems
  • The low air warning should activate before pressure drops below 60 psi in either system
  • The low air warning should activate before 120 psi since it's a dual air brake system
  • The low air warning should only activate when both systems drop below 60 psi

Quote From The CDL Manual:

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 brake system fixed.

TruckingTruth's Advice:

Regardless of whether your vehicle has a single air brake system or a duel air brake system, if any air pressure gauge reads below 60 psi, you should have a warning about low air pressure even if the second system is fully charged.

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When checking manual slack adjusters on S-Cam brakes, you should be sure the slack adjuster moves no more than about:
  • 1 inch
  • 1/4 inch
  • 1/2 inch
  • 3/4 inch

Quote From The CDL Manual:

Check manual slack adjusters on S-Cam brakes: Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjusting. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

TruckingTruth's Advice:

This is a very important figure to memorize. Remember, the slack adjuster, when properly adjusted, should move no more than 1 inch.

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Before driving a vehicle with a dual air brake system, the air pressure should be above what psi?
  • 80 psi
  • 120 psi
  • 60 psi
  • 100 psi

Quote From The CDL Manual:

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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What is a dual air brake system
  • Two separate air brake systems that use a single set of brake controls
  • A fail safe system where when one air brake system fails below 60 psi, the second system will keep the emergency brakes from activating
  • A single air brake system that uses two sets of brake controls
  • A system which does not have spring brakes and instead, uses a second S-Cam system for its emergency brakes

Quote From The CDL Manual:

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that 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.

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What are slack adjusters?
  • A broad, very short cylinder attached to a wheel, against which the brake shoes press in a drum brake
  • Material attached to a brake shoe to increase friction against the brake drum
  • The device which converts the air pressure force into a mechanical force
  • An adjustable member that transmits brake application force and permits compensation for lining wear

Quote From The CDL Manual:

Check manual slack adjusters on S-Cam brakes: Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjusting. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

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Next
When checking manual slack adjusters on S-Cam brakes, you should park on:
  • A slight decline
  • A slight incline
  • Level ground
  • Manual slack adjusters can only be checked with the wheels removed

Quote From The CDL Manual:

Check manual slack adjusters on S-Cam brakes: Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjusting. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

TruckingTruth's Advice:

Manual slack adjusters should only be checked while parked on level ground. Be sure to have this memorized as it will come up not only on the written exam, but possibly on your pre-trip exam as well.

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Are brake drums or discs allowed to be cracked?
  • No, brake drums or discs may not have any cracks at all
  • Yes, but they must not have cracks longer than one-half the width of the friction area
  • Yes, but they must not have cracks longer than one-third the width of the friction area
  • Yes, but they must not have cracks longer than three-quarters the width of the friction area

Quote From The CDL Manual:

Check brake drums or discs, linings and hoses: Brake drums or discs must not have cracks longer than one-half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.

TruckingTruth's Advice:

Obviously, you don't want brake drums or discs to have any cracks at all. But regulations allow cracks as long as they are under on-half the width of the friction area.

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While inspecting brake linings, all of the following statements are true except:
  • Be sure linings are not soaked with oil or grease
  • Linings must not be dangerously thin
  • Be sure linings have oil and grease on them to help prevent overheating
  • Linings must not be loose

Quote From The CDL Manual:

Check brake drums or discs, linings and hoses: Brake drums or discs must not have cracks longer than one-half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.

TruckingTruth's Advice:

Brake linings can become covered with oil, grease, or other foreign matter kicked up from the roadway surface. Since oil and grease will greatly reduce the stopping power, be sure your brake linings are free from this type of debris.

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While inspecting the air compressor, you should:
  • Visually check oil level inside the air compressor using a dip-stick
  • Check the condition and tightness of the belt (if belt driven)
  • Unbolt the air compressor from the engine as visually inspecting it while attached is nearly impossible
  • Take the front cover off so you can look inside the vacuum seal

Quote From The CDL Manual:

Check air compressor drive belt if compressor is belt driven. If the air compressor is belt driven, check the condition and tightness of the belt. The belt should be in good condition.

TruckingTruth's Advice:

Some air compressors are gear driven, in which case, no belt will be present. Otherwise, be sure to check the belt to be sure it isn't frayed, cracked, off its tracks, or otherwise broken.

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In a Dual Air Brake System, what are the proper system names?
  • The first system is called the Parent System and the other is called the Child System
  • The first system is known as System One and the other is called System Two
  • The first system is called the Primary System and the other is called the Secondary System
  • The first system is called the Main System and the other is called the Backup System

Quote From The CDL Manual:

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that 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.

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