Free CDL Air Brakes Practice Test

1. When you are testing the air leakage rate, the air loss rate after the initial pressure drop should be less than __________ psi in one minute for combination vehicles.

• A. two
• B. three
• C. four
• D. five

Explanation:
If you drive a vehicle with air brakes, you should conduct a final air brake check instead of a hydraulic brake check during step seven of the Seven-step Inspection Method. Test the air leakage rate. 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 psi in one minute for single vehicles and less than three psi in one 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 psi in one minute for single vehicles (more than four psi for combination vehicles), the air loss rate is too much. Check for air leaks and fix the problem before driving the vehicle. Otherwise, you could lose your brakes while driving.

In the handbook:
5.3.3: Inspecting Air Brake Systems - Step 7 Final Air Brake Check

2. Automatic front wheel limiting valves __________

• A. have a control marked "normal" and "slippery" that is controlled by the driver.
• B. reduce the air to the front brakes except when the brakes are used at 60 psi or more application pressure.
• C. are an electric switch that turns on the brake lights when you use the air brakes.
• D. both A and B

Explanation:
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 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.

In the handbook:
5.1.13: The Parts of an Air Brake System - Front Brake Limiting Valve

3. If there is an air leak in the system, a heavily loaded vehicle will take a long distance to stop because __________

• A. the spring brakes work on all axles.
• B. it will skid out of control when the spring brakes come on.
• C. controlled braking is not possible.
• D. none of the above

Explanation:
If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on. It is much safer to stop while there is enough air in the tanks to use the foot brakes.

In the handbook:
5.4.7: Using Air Brakes - Low Air Pressure

4. In air-operated disc brakes, the pressure of the brake chamber on the __________ turns the power screw.

• A. slack adjuster
• B. s-cam
• C. rotor
• D. brake chamber push rod

Explanation:
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. Disc brakes are less common than s-cam brakes.

In the handbook:
5.1.8: The Parts of an Air Brake System - Foundation Brakes

5. __________ allow you to release the spring brakes so you can move the vehicle in an emergency.

• A. Dual parking control valves
• B. Modulating control valves
• C. Front brake limiting valves
• D. none of the above

Explanation:
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.

In the handbook:
5.1.15: The Parts of an Air Brake System - Parking Brake Controls