DMV PA - CDL Air Brakes Endorsement Text Book Questions (And Answers)

  1. 1. Why must air tanks be drained?
    • 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, shown in figure 5-1, or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.

    • Automatic–the water and oil is automatically expelled. They may be equipped for manual draining as well.

    The automatic types are available with electric heating devices. These help prevent freeze up of the automatic drain in cold weather.

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  2. 2. What is a supply pressure gauge used for?

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

    • 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 cause by brakes out of adjustment, air leaks or mechanical problems.

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  3. 3. All vehicles with air brakes must have a low air pressure warning signal. True or False?


    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." 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.

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  4. 4. What are 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 40 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.

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  5. 5. Front wheel brakes are good under all conditions. True or False?

    • 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 vary hard (60 psi or more application pressure). These valves cannot be controlled by the driver.

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  6. 6. What is a dual air brake system?

    Most newer 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.

    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 brakes system fixed.

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  7. 7. What are the slack adjusters?
    • When you push the brake pedal, air is let into each brake chamber (see figure 5-2). Air pressure pushes the rod out, moving the slack adjuster (an adjustable device used to compensate for brake shoe wear), thus twisting the brake cam shaft. 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.

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    • "Q:What are Slack Adjusters?

    Slack adjusters are used to adjust for wear on s-cam brakes on heavy and medium size trucks."

  8. 8. How can you check slack adjusters?
    • 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 adjustment. Adjust it or have it adjusted. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspection. Be safe, check the slack adjusters.

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  9. 9. How can you test the low pressure warning signal?

    • Shut the engine off when you have enough air pressure so that the low pressure warning signal is not on. 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 (or tank with the lowest air pressure, in dual air systems).
    • If the warning signal doesn't work, you could lose air pressure and you would not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

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  10. 10. What can you check that the spring brakes come on automatically?
    • Chock the wheels, release the parking brakes when you have enough air pressure to do it, and shut the engine off. 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 (usually in a range between 20-40 psi). This causes the spring brakes to come on.

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  11. 11. What are the maximum leakage rates?

    With a fully-charged air system (typically 125 psi), turn off the engine, release the parking brake and time the air pressure drop. The loss rate should be less than two (2) psi in one minute for single vehicles and less than three (3) psi in one (1) minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than three (3) psi in one minute for single vehicles (more than 4 psi for combination vehicles), the air loss rate is too much. Check for air leaks and fix before driving the vehicle. Otherwise, you could lose your brakes while driving.

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  12. 12. What factors can cause brakes to fade or fail?

    Brakes are designed so brake shoes or pads rub against the brake drum or disks to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

    Excessive use of the service brakes result in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining which reduce friction and also causes expansion to the brake drums. As the overheated drums expand, the brake shoes and linings have to move farther to contact the drums, and the force of this contact is also reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

    • Brake fade is also affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.
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  13. 13. The use of brakes on a long steep downgrade is only a supplement to the braking effect of the engine. True or False?



    The use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

    • Apply the brakes just hard enough to feel a definite slowdown.

    • When your speed has been reduced to approximately five (5) mph below your "safe" speed, release the brakes. (This brake application should last for about three (3) seconds.)

    • When your speed has increased to your "safe" speed, repeat steps 1 and 2.

    For example, if your "safe" speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this action as often as necessary until you have reached the end of the downgrade.

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  14. 14. If you are away from your vehicle only a short time, you don't need to use the parking brake. True or False?


    Any time you park, use the parking brakes, except as noted below. Pull the parking brake control knob out to apply the parking brakes, push it in to release them. The control will be a yellow, diamond-shaped knob labeled "parking brakes" on newer vehicles. On older vehicles, it may be a round blue knob or some other shape (including a lever that swings from side to side or up and down).

    Don't use the parking brakes if the brakes are very hot (from just having come down a steep grade) or if the brakes are very wet in freezing temperatures. If they are used while they are very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks to hold the vehicle. Let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.

    If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

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  15. 15. How often should you drain air tanks?
    If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil.

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Card Set
DMV PA - CDL Air Brakes Endorsement Text Book Questions (And Answers)
Test Questions (and answers,) for the Air Brakes Endorsement portion of the Commercial Drivers License Exam of Pennsylvania, but taken directly from the Commercial Drivers Manual.