Mississippi CDL: Air Brakes Practice Test

Not quite. The answer is press the brake pedal as hard as you can, release the brakes when the wheels lock, and apply the brakes again when the wheels start to roll.

To stop with stab braking, brake hard, release the brakes once the wheels lock, and then brake hard once the wheels start to roll again. (Note that this method won't work if your vehicle has antilock brakes, because the wheels won't lock. Use controlled braking instead.)

Not quite. The answer is does all of the above.

The tractor protection valve protects your air brake system by providing the air supply, closing automatically if the air supply drops too low, and closing if you apply the parking brakes.

Not quite. The answer is the brakes to fade.

Excessive heat caused by overuse of the brakes can expand the brake drums away from the brake pads or shoes and affect the brake linings, reducing friction. The brakes will fade (lose their effectiveness). On a long or steep downgrade, rely on engine braking more than the service brakes.

Not quite. The answer is lets the air out of the brake chambers.

When you apply the parking brake control, air is released from the brake chambers, causing the springs to apply the brakes and hold your vehicle in place.

Not quite. The answer is It is exhausted from the system.

Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor.

Not quite. The answer is that you are applying to the brakes.

If your vehicle is so equipped, the application pressure gauge will show how much air pressure you are applying to the brakes. If you have to apply more pressure than before to achieve the same braking effect, your brakes are fading (becoming less effective).

Not quite. The answer is (b) or (c)

Before measuring pushrod stroke, chock the wheels of your vehicle. For an accurate measurement, make sure that tank air pressures are 90–100 psi. Here are two different methods for measuring pushrod stroke: (1) Make a mark on the pushrod where it exits the brake chamber, and see how far it moves from the brake chamber when the service brakes are applied. (2) Measure the length of the part of the pushrod from the brake chamber to the center of the clevis pin, and see how much it increases when the brakes are applied. To perform either method efficiently, two people may be needed: one to apply the service brakes, and one to make the appropriate measurements before and after the brakes are applied. There are also various pushrod stroke measuring tools available commercially that you can use.

Not quite. The answer is 10

A CMV has two foundation brakes per axle (one on each side). A typical 18-wheeler has five axles: the steering axle, the two tandem drive axles, and the two tandem semi-trailer axles. Therefore, there are 5 × 2 = 10 foundation brakes on the vehicle. Note: According to FMCSA, even a lift axle must have brakes when it is in contact with the roadway.

Not quite. The answer is Otherwise, you will have to get a CDL with restrictions.

If you do not pass the air brake knowledge test, you will only be able to get a CDL with a restriction that limits you to driving vehicles without air brakes. So, good luck!

Not quite. The answer is Stop and park safely.

If the low air pressure warning comes on, there may be a leak in the system. Pull over and park your vehicle as soon as possible. If the air pressure drops too low, the spring brakes will come on automatically, which will stop your vehicle – but not necessarily in a safe way. Sudden application of the spring brakes while you're driving might result in a skid.

Not quite. The answer is 8

A CMV has two foundation brakes per axle (one on each side). According to FMCSA, even a lift axle must have brakes when it is in contact with the roadway. Therefore, a truck with 4 axles must have 4 × 2 = 8 foundation brakes, even if one axle is a lift axle.

Not quite. The answer is a supply pressure gauge.

Every vehicle with air brakes has a supply pressure gauge, which shows how much air pressure is in the air tanks. Some vehicles also have an application pressure gauge, which shows how much air pressure you are applying to the brakes. Older vehicles also had a manual front brake limiting valve, which allowed the driver to cut front brake air pressure in half as a way of preventing front-wheel skids. However, tests have shown that full front-wheel braking is safe.

Not quite. The answer is compressed air enters the brake chambers.

Pressing down on the brake pedal lets air into each brake chamber, forcing the brake pads against the inside of the drum (or the rotor for disc brakes).

Not quite. The answer is Apply the brakes as hard as you can without locking the wheels.

In controlled braking, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. This method of emergency braking does not require you to lock the wheels. Hence it is compatible with antilock brakes (ABS).

Not quite. The answer is 60

The low air pressure warning light must come on before the air pressure falls below 60 psi. On some vehicles, the warning light will come on at a higher air pressure. Check the manufacturer's specifications.

Not quite. The answer is At the end of each working day

Unless your vehicle automatically drains the air tanks, you should drain them at the end of each working day so that the moisture and oil will not freeze or coagulate and cause your brakes to fail.

Not quite. The answer is Stop and park safely. Continue only after the system has been fixed.

A dual air brake system has two separate air brake systems, each with its own air tanks, hoses, lines, etc. Typically, one system operates the brakes on the rear axle(s), and the other system operates the brakes on the front axle. If one of the systems loses air pressure (causing the low air pressure warning), either the front or the rear brakes won't work properly. As a result, your vehicle's braking distance will increase. If this happens, stop your vehicle and get the system fixed.

Not quite. The answer is brake drum.

In North America, the most common type of foundation brake is the S-cam drum brake. The brake shoes and linings press against the inside of the brake drum, which causes friction to slow the wheel.

Not quite. The answer is a yellow lamp on the left side.

A converter dolly with antilock brakes is required to have a yellow ABS lamp on its left side. All converter dollies built since March 1, 1998 have antilock brakes.

Not quite. The answer is Before driving

Air compressors are either belt driven or (as is more often the case these days) gear driven directly from the engine. Most modern compressors are cooled by the engine cooling system and are lubricated by engine oil. However, some compressors may still be self lubricated or air cooled. If the compressor has its own oil supply, check the oil level before you drive.