CDL Air Brakes Practice Test 5


Which of the following steps should you use to check the slack adjusters on S Cam brakes?

Correct! Wrong!

Use gloves and pull hard on each slack adjuster. You should park on level ground and chock the wheels. The parking brakes need to be off so that you can move the slack adjusters. Using gloves, you should pull hard on each slack adjuster. If the slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.

What is the typical air pressure of a fully charged air brake system?

Correct! Wrong!

125 psi. A fully charged air system typically has 125 psi of air pressure.

What is the proper way to test the air leakage rate?

Correct! Wrong!

Turn off the engine and release the parking brake. You should test for air leakage with a fully-charged air system. You should turn off the engine, release the parking brake, and time the air pressure drop. 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.

What is stab braking?

Correct! Wrong!

Applying your brakes until the wheels lock up, releasing, and then reapplying when the wheels start rolling. Controlled braking is applying the brakes as hard as possible without locking the wheels. Both controlled braking and stab braking are effective ways to brake that will keep your vehicle in a straight line and allow you to turn if necessary.

Why do air brakes take longer to stop than hydraulic brakes?

Correct! Wrong!

It takes longer for the air to flow through the lines. Hydraulic brakes (used on cars and light trucks) work instantly. Air brakes have an added delay because of the time required (about 1/2 second) for the air to flow through the lines.

What will happen if your air compressor stops?

Correct! Wrong!

You'll still have air pressure left in the air tanks to stop. If your compressor stops working, you will still have air pressure left in your tanks, but the air compressor will not be able to replace air lost by using your brakes.

How should you check your air compressor governor "cut in" and "cut out" pressures?

Correct! Wrong!

All of the above. To check your air compressor governor cut-in and cut-out pressures you should run the engine at a fast idle and monitor the air pressure gauge to see if the air governor cuts out at the manufacturer's specified pressure (typically about 125 psi). With the engine idling, you should step on and off the brake to reduce tank pressure and see if the governor cuts in at the manufacturer's specified pressure (typically about 100 psi).

How fast should the air pressure build in a dual air system?

Correct! Wrong!

From 85 to 100 psi in 45 seconds. In a single air system, the pressure should build from 50 to 90 psi in 3 minutes.

What are the typical air compressor governor cut-in and cut-out pressures?

Correct! Wrong!

Cut-in: 100 psi; Cut-out: 125 psi. When air tank pressure rises to the cut-out level (around 125 psi) the governor stops the compressor from pumping air. When the tank falls below the cut-in pressure (around 100 psi) the governor allows the compressor to start pumping again.

What device will keep air in the tanks if the air compressor develops a leak?

Correct! Wrong!

The one way check valve. The one way check valve sits between the air compressor and the main air tanks. It allows air flow in one direction only, which prevents leaks in the air compressor from draining the air tanks. The trailer protection valve's purpose is to keep air in the brake system in case the trailer breaks away or develops a bad leak. A safety relief valve is installed in the first tank that the air compressor pumps air into. A safety valve protects the tank and system from too much pressure by opening up at 150 psi. The treadle valve is another name for the foot pedal.

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