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5.1 Parts of Air Brake System (continued from previous page)

Supply Pressure Gauges

All air-brake vehicles have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). These gauges tell you how much pressure is in the air tanks.

Application Pressure Gauge

This gauge shows how much air pressure you are applying to the brakes (this gauge is not on all vehicles). Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure also can be caused by brakes out of adjustment, air leaks or mechanical problems.

Low Air-Pressure Gauge

A low air-pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer also may come on.

Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the “out of view” position manually. It will not stay in place until the pressure in the system is above 60 psi.

On large buses it is common for the low-pressure warning devices to signal at 80-85 psi.

Stop Light Switch

Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.

Front Brake Limiting Valve

Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked “normal” and “slippery.” When you put the control in the “slippery” position, the limiting valve cuts the “normal” air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front-wheel braking is good under all conditions. Tests show that front-wheel skids from braking are not likely even on ice. Make sure the control is in the “normal” position to have normal stopping power.

Many vehicles have automatic front-wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). These valves cannot be controlled by the driver.

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 lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.

Tractor and straight truck spring brakes will come on fully when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 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.

Parking Brake Controls

In newer vehicles with air brakes, you apply the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes (spring brakes) on, and push it in to release them. On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park.

Caution: Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. But not all systems are set up that way, and those that are may not always work. It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on.

Modulating control valves:

In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a “modulating valve.” It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.

Dual parking control valves:

When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank that can be used to release the spring brakes so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring-loaded in the “out” position. When you push the control in, air from the separate air tank releases the spring brakes so you can move. When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times. Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out.

Remember: Supply Pressure Guages tell you how much pressure is in the air tanks. This should be memorized!

Once again, you need to memorize this!

The Application Pressure Gauge tells you how much air pressure you are applying to the brakes.

The harder you press the brake pedal, the higher the pressure. While not all air brake vehicles have this guage, most modern combination vehicles will have one.

There are a couple key points about the low air-pressure gauge that you need to remember:

  • A low air pressure warning signal is required on all vehicles with air brakes.
  • A visual warning signal must come on before the air pressure in the tanks falls below 60 psi.

You need to memorize that the warning signal must come on before 60 psi. That's very important.

It is very unlikely you will ever see a wig wag as they are only used on older vehicles. But there may still be a question about this on the written exam. The key point to remember is that the wig wag device must come into view when pressure drops below 60 psi.
Be sure you understand the concept of spring brakes, which are required on all vehicles using an air brake system. The brakes naturally want to be engaged. The only reason the brakes are not engaged is because air pressure keeps them disengaged. If air pressure is lost, the brakes will engage automatically.

It is very important you memorize these psi ranges as this will come up many times. To help simply, here are the psi ranges you need to memorize:

  • Before air pressure drops to 60 psi, a low air pressure warning signal must activate.
  • When air pressure drops to between 20 and 45 psi, the vehicles spring brakes will automatically engage.
Remember: The parking brakes are controlled using a diamond-shaped, yellow, push-pull control knob. You may be asked about this on the written exam.
While it may appear to be common sense, questions on the written exam frequently ask about the use of parking brakes. Simply remember that parking brakes need to be used anytime your vehicle is parked.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

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

What is the supply pressure gauge used for?
  • Indicates how much air pressure is in the air holding tanks
  • Indicates how much pressure is being supplied to the emergency brake
  • Indicates how much pressure is being supplied to the parking brake
  • Indicates how much pressure is being supplied to the service brake

Quote From The CDL Manual:

Supply Pressure Gauges - All air-brake vehicles have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). These gauges tell you how much pressure is in the air tank

Next
What is a Modulating Control Valve?
  • A valve which allows you to use the spring brakes for everyday driving
  • A brake valve which allows the truck to roll back, but not forwards
  • A control handle on the dash board may be used to apply the spring brakes gradually
  • A valve used to apply the service brakes

Quote From The CDL Manual:

Modulating control valves: In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a "modulating valve." It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.

TruckingTruth's Advice:

Very few vehicles are equipped with modulating control valves. But since there are some air brake vehicles with this valve, you may be asked a question about it on the written exam.

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Next
When will spring brakes automatically activate?
  • When air pressure drops to a range of 20 to 45 psi
  • When air pressure drops to a range of 100 to 120 psi
  • When air pressure is completely depleted
  • Spring brakes should never activate automatically

Quote From The CDL Manual:

Tractor and straight truck spring brakes will come on fully when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 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.

TruckingTruth's Advice:

Remember, spring brakes are used as both parking brakes as well as emergency brakes. If you begin losing air pressure, it is important to pull over quickly to a safe area as your spring brakes will automatically activate once air pressure drops too low.

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Next
Which of these statements about the Low Air-Pressure Gauge is false?
  • A low pressure warning system is optional on vehicles with air brakes
  • On older vehicles, the warning signal will come on at one-half the compressor governor cutout pressure
  • A warning signal must come on before the air pressure in the tanks falls below 60 psi
  • On large buses it is common for the low-pressure warning devices to signal at 80 85 psi

Quote From The CDL Manual:

Low Air-Pressure Gauge - A low air-pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer also may come on.

TruckingTruth's Advice:

The most important thing to pay attention to here is that the warning indicator must come on before air pressure falls below 60 psi. That is a number you'll see over and over again so it's crucial you have that memorized. You will use that for the written test, the pre-trip inspection exam, as well as in your day to day job activities.

Prev
Next
Which of the following is true about Supply Pressure Gauges?
  • All air-brake vehicles have a pressure gauge connected to the air tank
  • All of these answers are true
  • These gauges tell you how much pressure is in the air tanks
  • If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles)

Quote From The CDL Manual:

All air-brake vehicles have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). These gauges tell you how much pressure is in the air tanks.

TruckingTruth's Advice:

Nearly all combination vehicles run on dual air brake systems. Be sure to locate where the gauges are on the dash as well as where the low air pressure warning light is located.

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Next
What are Spring Brakes?
  • An S-Cam forces the brake shoes away from one another and presses them against the inside of the brake drum
  • Powerful springs are held back by air pressure and if the air pressure is removed, the springs put on the brakes
  • Air pressure acts on a brake chamber and slack adjuster
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes

Quote From The CDL Manual:

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 lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.


Tractor and straight truck spring brakes will come on fully when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 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.

TruckingTruth's Advice:

Spring brakes control your parking and emergency brakes. It's a fail-safe system where brakes are held off with air pressure. If you lose air pressure, the brakes will no longer be held back and will automatically activate. That is why it's important to pay attention to your air pressure gauges. If your air pressure drops too low, you need to pull over before the emergency brakes (spring brakes) activate.

Prev
Next
Which of the following statements about Parking Brake Controls is false?
  • On older vehicles, the parking brakes may be controlled by a lever
  • You apply the parking brakes using a circle-shaped, red knob
  • You pull the knob out to put the parking brakes (spring brakes) on.
  • All of these statements are true

Quote From The CDL Manual:

Parking Brake Controls - In newer vehicles with air brakes, you apply the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes (spring brakes) on, and push it in to release them. On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park.

Caution: Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. But not all systems are set up that way, and those that are may not always work. It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on.

Prev
Next
What is a Wig-Wag?
  • In older vehicles, this is a mechanical type of low air pressure warning indicator
  • A new technology using GPS which automatically fills the air tanks when a steep decline is ahead
  • It is located inside of the brake chamber and records any mechanical failures
  • This is a type of supply pressure gauge which normally hangs from near the visor

Quote From The CDL Manual:

Another type of warning is the "wig wag." This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the "out of view" position manually. It will not stay in place until the pressure in the system is above 60 psi.

TruckingTruth's Advice:

Except in very old commercial vehicles, the Wig-Wag warning device is about extinct and is no longer used. Simply know what it is as there may be a question asked about it on the written exam. It is likely you will never encounter a wig wag warning device in real life.

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Next
In a vehicle equipped with an air brake system, what should happen when air pressure drops below 60 psi
  • The emergency brakes should activate
  • Nothing, 60 psi is considered "normal operating range"
  • None of these answers are correct
  • The service brakes will begin to slowly apply brake pressure

Quote From The CDL Manual:

A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles).

TruckingTruth's Advice:

Before air pressure drops below 60 psi, the low air warning signal will activate. When the warning activates, you need to quickly find a safe place to pull your vehicle to the side of the road. If you wait too long, the emergency brakes will activate once your air pressure reaches 45 psi and you may not be able to clear the roadway.

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Next
What is a Front Brake Limiting Valve?
  • When you put the control in the "slippery" position, the limiting valve cuts the "normal" air pressure to the front brakes by half
  • All of these accurately describe a Front Brake Limiting Valve
  • Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces
  • These are only installed on some vehicle made before 1975

Quote From The CDL Manual:

Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked "normal" and "slippery." When you put the control in the "slippery" position, the limiting valve cuts the "normal" air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front-wheel braking is good under all conditions. Tests show that front-wheel skids from braking are not likely even on ice. Make sure the control is in the "normal" position to have normal stopping power.

Many vehicles have automatic front-wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). These valves cannot be controlled by the driver.

TruckingTruth's Advice:

It is extremely unlikely you will ever drive a vehicle with front brake limiting valves. However, there is a chance you may be asked a question about this on the written exam.

Prev
Next
Which statement best describes the Application Pressure Gauge?
  • Shows how much air pressure you are applying to the brakes
  • Application pressure gauges must be installed on all commercial vehicles
  • Let's you know when air pressure in the tanks are too low
  • Tells you how much pressure is in the air tanks

Quote From The CDL Manual:

Application Pressure Gauge - This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure also can be caused by brakes out of adjustment, air leaks or mechanical problems.

Prev
Next
At what psi must the low air pressure warning indicator activate?
  • Before dropping below 40 psi
  • Before dropping below 20 psi
  • Before dropping below 60 psi
  • Before dropping below 80 psi

Quote From The CDL Manual:

A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles).

TruckingTruth's Advice:

On large busses, it is common for the low-pressure warning devices to signal at 80 to 85 psi, but that is an additional safety feature. Regulations require the warning to activate before 60 psi. This is very important to memorize not only for testing, but for everyday job duties.

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