Air brakes- CDL Permit

Why must air tanks be drained?

Air tanks must be drained because compressed air contains water and compressor oil, which collect at the bottom of the tank. Water and oil can freeze, corrode the system, and cause complete brake failure.

What is a supply pressure gauge used for?

• Dual Systems: If your vehicle has a dual air brake system, the gauge will have two needles (or two separate gauges)—one for System 1 (primary) and one for System 2 (secondary).

• Normal Operating Range: During normal driving, supply pressure should read between 100 and 125 psi (pounds per square inch).

• Compressor Cut-Off: The air compressor governor should cut out at around 120 to 145 psi when the tanks are fully charged.

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

True

What are spring brakes?

Spring brakes are powerful mechanical springs used as emergency brakes and parking brakes. They are held in the released position by air pressure; if air pressure is lost, the springs automatically apply the brakes.

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

True

How do you know if your vehicle is equipped with antilock brakes? 

You can tell if your vehicle has ABS by checking for a yellow ABS warning light on the instrument panel, checking for a yellow ABS indicator light on the left rear side of a trailer, or looking under the vehicle for wheel speed sensor wires leading to the brake assemblies.

What is a dual air brake system?

A dual air brake system has two separate air brake systems that use a single set of brake controls. One system operates the primary brakes (usually the rear axle/drive axles) and the other operates the secondary brakes (usually the front axle/steering axle).

What are the slack adjusters?

Slack adjusters are mechanical levers that connect the brake chamber's push rod to the brake camshaft. They adjust the distance (slack) between the brake shoes and the brake drum as the brake linings wear down.

How can you check slack adjusters?

To check slack adjusters, park on level ground, chock the wheels, and release the parking brakes. Pull hard on each slack adjuster by hand; if it moves more than about one inch where the push rod attaches to it, it needs adjustment.

How can you test the low pressure warning signal?

To test the low air pressure warning signal, you must turn the engine off with the ignition key in the "ON" position, pump (fan) the foot brake pedal to bleed down air pressure, and verify that the low air pressure warning light and/or buzzer turns ON at or before 60 psi.

How can you check that the spring brakes come on automatically?

To check that the spring brakes come on automatically, you must chock the wheels, release the parking brakes, turn off the engine, pump the foot brake pedal to bleed down air pressure, and verify that the parking brake valve (yellow knob) pops out automatically between 20 and 45 psi.

What are the maximum leakage rates?

single vehicles: Static 2 psi per min and Applied 3 psi per min

Combo vehicles: Static 3 psi per min and Applied 4 psi per min

Why should you be in the proper gear before starting down a hill?

You must shift into a low gear before starting down a hill because you may not be able to shift into a lower gear once you are moving downhill. If you try to shift on the grade, you can get caught in neutral, lose engine braking entirely, and cause a dangerous runaway truck situation.

What factors can cause brakes to fade or fail?

• Overusing the service brakes instead of using engine braking on downgrades.

• Brakes being out of adjustment (which causes the remaining working brakes to overheat rapidly).

• Excessive speed on long, steep downgrades.

If you are away from your vehicle only a short time, you do not need to use the parking brake. True or False?

False

How often should you drain air tanks?

You should drain manual air tanks at the end of every driving day (or daily) to remove accumulated water and compressor oil.

How should you brake when you drive a tractor-trailer combination with ABS?

You should brake normally—just as you would without ABS. Apply the brakes with the amount of pressure necessary to stop safely, and maintain firm pressure. Do not pump the brakes on an ABS-equipped vehicle.

You still have normal brake functions if your ABS is not working. True or False?

True