Air Brakes test at CDL School.

Air Brakes test at CDL School.

How to Pass Air Brakes Test at DMV?

One of the fastest ways to pass the Walk Around, Skills Test, and Road Trip where you prove to the DMV that you know how to operate an Air Brakes system on a semi-tractor and trailer.  The DMV is guaranteed to ask questions about Air Brakes Components so know all the pictures and names of components.

Also, when talking to the DMV about Air Brakes you can memorize the 7 Step Air Brake Check.  If you say it exactly or close to it, they will pass you for DMV air brakes knowledge.  You can find the 7 Step Air Brakes Check Here.

How to Pass an Air Brakes Test at the DMV?

DMV Skills TEST

DMV CDL Skills Test - Pre-Trip Inspection, walk-around, Road Trip.

First download the CDL HANDBOOK

Know any images / photos with different air brakes parts named.  Be prepared to name each Air Brake part. We provide free online CDL practice tests for air brakes endorsement test. Try our free Air Brakes practice test so you know how the website works.

Then sign up as a full member to learn about your state’s local DMV air brakes test questions.  You can complete the FREE CDL practice test as many times as needed.  But do not forget to sign up for the full membership to get over 2,000 DMV practice test questions and it covers the air brakes components thoroughly.

If you would like to learn more about Air Brakes Components Please SEE the list of air brakes components below:

There are many parts to a tractor-trailer air brake system.

It is important to understand all air brakes parts and what they do.

  1. Air Brakes - Air Compressor

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a v-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

  1. Air Brakes - Air Compressor Governor

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (around 125 pounds per-square-inch or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

  1. Air Brakes - Air Storage Tanks

Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.

  1. Air Brakes - Air Tank Drains
Air tank with Manual Draining Valve for Air Brakes Test - CDL.

Air tank with Manual Draining Valve for Air Brakes Test - CDL.

Figure 4-1 Air Tanks Drains using the Manual Draining Valve on bottom of the  Air Tank.

Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types:

  • Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving. See Figure 4.1.
  • Automatic--the water and oil are automatically expelled. These tanks may be equipped for manual draining as well.

Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.  See Figure 4.1.

  1. Air Brakes Alcohol Evaporator

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary, every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil. (Unless the system has automatic drain valves.)

  1. Air Brakes Safety Valve

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.

  1. The Air Brake Pedal

You put on the brakes by pushing down the brake pedal. (It is also called the foot valve or treadle valve.) Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes won't work.

  1. Foundation Air Brakes – the drum brake air brake component list
Air Brake Test CDL - Drum Brake picture with list of parts.

Air Brake Test CDL - Drum Brake picture with list of parts.

Figure 8-2 Drum Brake Air Brakes Parts List must know on the CDL TEST for Air Brakes Endorsement and on the DMV Skills Test, Road Trip and Pre-trip parts of DMV TEST.

Foundation brakes are used at each wheel. The most common type is the s-cam drum brake. The parts of the brake are discussed below.

Brake Drums, Shoes, and Linings. Brake drums are located on each end of the vehicle's axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.

S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the s-cam (so called because it is shaped like the letter "S"). The s-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the s-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again. See Figure 8.2.

Wedge Brakes. In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber, or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge type brakes may be self-adjusting or may require manual adjustment.

Disc Brakes. In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like s-cam brakes. But instead of the s-cam, a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large c-clamp.

Wedge brakes and disc brakes are less common than s-cam brakes.

  1. Air Brakes - Supply Pressure Gauges

All vehicles with air brakes 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.) Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.

  1. Air Brakes - 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 can also be caused by brakes out of adjustment, air leaks, or mechanical problems.

  1. Air Brakes - Low Air Pressure Warning

You must memorize how the low air pressure warning works.  It is not only on the DMV CDL Tests but also a matter of life and death when considering safety.  We strongly recommend that you have access to real tractor-trailers to practice using the air brakes.

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 may also 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.

  1. Ari Brakes - 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.

  1. 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 have shown 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.

  1. 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 fully on 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.

  1. Parking Brake Controls for Air Brakes on Tractor Trailers.
Tractor Protection Valve Air Brake Component for CDL TEST

Tractor Protection Valve Air Brake Component for CDL TEST

Figure 15-3 Tractor Parking Brake Control Knobs

In newer vehicles with air brakes, you put on 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 which can be used to release the spring brakes. This is 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. See Figure 15.3.

You must know the names of the knobs and how they all work.  It is extremely important part of being a professional truck driver so get ready to memorize it all and learn it.  If you do not learn the air brakes components prior to driving a commercial vehicle you are considered an unsafe driver who needs more training.  See Figure 15.3.

  1. Semi-Tractor & Trailer - Antilock Braking Systems (ABS)

Truck tractors with air brakes built on or after March 1, 1997, and other air brakes vehicles, (trucks, buses, trailers, and converter dollies) built on or after March 1, 1998, are required to be equipped with antilock brakes. Many commercial vehicles built before these dates have been voluntarily equipped with ABS. Check the certification label for the date of manufacture to determine if your vehicle is equipped with ABS. ABS is a computerized system that keeps your wheels from locking up during hard brake applications.

Vehicles with ABS have yellow malfunction lamps to tell you if something isn't working.

Tractors, trucks, and buses will have yellow ABS malfunction lamps on the instrument panel.

Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner. Dollies manufactured on or after March 1, 1998 are required to have a lamp on the left side. On newer vehicles, the malfunction lamp comes on at start-up for a bulb check, and then goes out quickly. On older systems, the lamp could stay on until you are driving over five mph.

If the lamp stays on after the bulb check, or goes on once you are under way, you may have lost ABS control at one or more wheels.

In the case of towed units manufactured before it was required by the Department of Transportation, it may be difficult to tell if the unit is equipped with ABS. Look under the vehicle for the electronic control unit (ECU) and wheel speed sensor wires coming from the back of the brakes.

Air Brake System Components

Air Brake System Components

Figure 16-4 Air Brakes System Components and Location

ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up.  The computer takes control of braking by rapidly apply brakes pressure.

ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.  Most notably it helps to keep the tractor-trailer moving in a straight line during hard braking.  You may feel a strong vibration on the brake pedal when ABS is engaged.

FREE CDL TEST QUESTIONS
CDL-Test sample test questions from DMV Handbook:

  1. Why must air tanks be drained?
  2. What is a supply pressure gauge used for?
  3. All vehicles with air brakes must have a low air pressure warning signal. True or False?
  4. What are spring brakes?
  5. Are front wheel brakes good under all conditions? True or False?
  6. How do you know if your vehicle is equipped with antilock brakes?

According to the DMV Handbook these questions may be on your

CDL AIR BRAKES test.

Get the answers to these questions and more by reading this web page and memorizing the pictures and names of Air Brakes components.  Be prepared to understand the air brake parts on both the CDL Air Brakes test questions and demonstrated knowledge of the air brakes and how they work on the Road Trip and Skill Test and Pre-Trip.

More Free CDL TEST Questions for Air Brakes:

FREE CDL Air Brakes Test Questions
1.  It is mandatory that all vehicles with air brakes have a low air pressure warning signal.
*A. True
B. Only some vehicles
C.  False

2.  You can check slack adjusters by ______?
A. By using an adjustable wrench
*B. By pulling hard on each slack adjusters
C. By frequently checking the low pressure gauge
D. You cannot manually check slack adjusters

3.  By using an adjustable wrench?
A. The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles
*B. The loss rate should be less than three psi in one minute for single vehicles and less than five psi in one minute for combination vehicles
C. There should be no air leaks
D. Breaking on a long steep downgrade should be used only as a supplement to the braking effect of the engine.
E. Depends on the weight of the GVCW of the vehicle

4.  Air tanks should be drained_____.

A. Every 4 hours or 350 miles
B. Every 3 hours or 100 miles
C. Drain your air tanks at the end of each working day
D. Once a week
*E. None of the above

5.   A supply pressure gauge is used for ______.

*A. The gauges tell you how much pressure is in the air tanks
B. The gauges tell you how much pressure is in the air lines
C.  The gauges warn you about brake chamber leaks only
D. The gauges monitor you how much slack is in the slack adjusters

6. What is the reason that air tanks must be drained?
A.  To keep the tanks cool while not in use
*B. To keep oil and water from building up in the tank
C. Air tanks should never be drained
D. To keep oil and toxic gas from building up in the tank
E. All of the above

7. Which of the following factors is known to cause brakes to fade or fail? (select all that apply)
A.  Using the engine brake and not using the brakes often enough can cause them to fail
B. Mismatched retread tires on a trailer
*C. An unbalanced load with too much weight on any one axle
*D. Brakes can fade or fail from excessive heat caused by using them too much

8. You don't need to use the parking brake if you are away from your vehicle for only a short time.
A. True
*B. False

9.   ______ is known as slack adjusters?

A. Lever connecting the brake chamber push rod with the return spring
B. Lever connecting the brake shoe with the brake drum
C. Lever connecting the brake chamber with the brake camshaft
*D. Lever connecting the brake chamber push rod with the foundation brake camshaft

10.  Front wheel brakes are good to use under all conditions?
*True
False

11. Spring brakes are ______?

*A. Powerful springs that are held back by air pressure. If the air pressure is removed, the springs put on the brakes.
B.  Big springs full of air pressure. If the air pressure is increased, the springs put on the brakes.
C. Large springs forced closed by air pressure. If the air pressure is increased, the springs release on the brakes.
D. None of the above

12. Why should you be in the proper gear before starting down a hill?
A. It is better than being in an improper gear
*B. Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect
C. Because it helps build up air pressure in your air tanks
D. Water can build up in your air tanks by not relying on the engine braking

13.  A dual air brake system is best described as______?
A. dual air brake system has two separate air brake systems; the first system is called the primary system. The other is called the secondary system.
B. dual air brake system has two separate air brake systems which use a single set of brake controls
C. dual air brake system has two separate air brake systems, both systems supply air to the trailer if there is one.
D. dual air brake system has two separate air brake systems, each system has its own air tanks, hoses, lines, etc.
*E. All of the above

14. What is the method used to check that the spring brakes will come on automatically?
*A. Step on and off the brake pedal to increase the air tank pressure, the parking brake knob should pop out when the air pressure rises to the manufacturer's specification.
B. Step on and off the brake pedal to reduce the air tank pressure while moving at a high rate of speed, the parking brake knob should pop out when the air pressure falls to the manufacturer's specification locking up all your tires.
C. Step on and off the brake pedal to reduce the air tank pressure, the parking brake knob should pop out when the air pressure rises to the manufacturer's specification of 120 psi.
D. Step on and off the brake pedal to reduce the air tank pressure, the parking brake knob should pop out when the air pressure falls to the manufacturer's specification.
*E. Turn the electrical power on and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank.
F. Turn the electrical power off and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 30 psi in the air tank.
G. Turn the electrical power on and step on and off the brake pedal to increase air tank pressure. The low air pressure warning signal must come on before the pressure increases to more than 60 psi in the air tank.
H. Turn the electrical power off and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 90 psi in the air tank.