Missouri CDL: Air Brakes Practice Test

Not quite. The answer is your brakes may fail because of water freezing.

Oil and water can start to collect at the bottom of an air tank, which can lead to brake failure. Unless your vehicle automatically drains the air tanks, you should drain them at the end of each working day so the moisture and oil will not freeze or coagulate and cause your brakes to fail.

Not quite. The answer is stop braking, turn quickly, and countersteer.

To recover from a drive-wheel braking skid, first stop braking. The rear wheels will unlock and roll again so there will be no more tendency to slide sideways. Then quickly steer your vehicle back on course. You may find that your vehicle will overshoot and keep on turning, possibly skidding in the opposite direction. Countersteer to counteract this. Note: This maneuver takes some practice. You should become proficient in it before you actually need to use it.

Not quite. The answer is Turn the adjusting nut to move the end of the slack adjuster until the brake shoes contact the inside of the brake drum. Then turn the nut the other way to move the brake shoes away from the brake drum.

To adjust a manual slack adjuster, turn the adjusting nut so that the S-cam rotates in the same direction as when the brakes are applied. Keep turning the nut until the nut stops turning. At this point, the brake shoes should be pressing tight against the inside of the brake drum. Then turn the nut the other way about one-third of a turn to move the brake shoes away from the inside of the brake drum. Note: This procedure may not be described in your CDL manual.

Not quite. The answer is all of the above.

Spring brakes are an important backup system: powerful springs that automatically apply the brakes if they sense that air pressure has been lost for some reason, such as a leak. They will also apply the brakes if the psi gets too low, although ideally, you should take control of your brakes before that happens.

Not quite. The answer is it may lead to sudden emergency braking in a single-circuit air system.

The low pressure warning signal should come on before the air pressure falls below 60 psi. If it fails to do so, the air pressure may keep falling without your knowing it. In a single-circuit air system, your spring (emergency) brakes may come on suddenly. In a dual air system, either the front or the rear brakes won't work properly, the braking distance of your vehicle will increase, and continued braking will soon cause the spring brakes to come on.

Not quite. The answer is 55 to 60 psi.

A low pressure warning signal visible to the driver must come on before the pressure in the service air tank falls below 55 psi (or one-half the air compressor governor cutout air pressure on older vehicles).

Not quite. The answer is one-half

During your walk-around inspection, check the brake drums or brake discs to be sure that no cracks are longer than half the width of the friction area.

Not quite. The answer is Bring the vehicle to a safe stop and continue only after the system is fixed.

A dual air brake system has two separate air brake systems. Typically, the primary system operates the brakes on the rear axle(s), and the secondary system operates the brakes on the front axle (and possibly one rear axle). If one of these two 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 park on level ground, chock the wheels, and release the parking brakes.

Chock the wheels so you can release the parking brakes safely. Then put on gloves and pull hard on each slack adjuster you can reach. If it moves more than about one inch, it needs to be adjusted.

Not quite. The answer is All vehicles with air brakes currently in operation must have low air pressure warning signals.

All vehicles on the road that have air brakes must have low air pressure warning signals.

Not quite. The answer is In the first tank to which the air compressor pumps air

A safety relief valve is installed in the first tank that the air compressor pumps air to. This tank is known as the supply or "wet" tank. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi.

Not quite. The answer is It depends on the adjustment of the service brakes.

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the service brakes nor the emergency or parking brakes will work right.

Not quite. The answer is glad hands

As part of your vehicle inspection test, if your vehicle is equipped with air brakes and has a trailer, you will inspect the air connections between the truck or tractor and the trailer. These connections are made with hose couplers (commonly called "glad hands" because they vaguely resemble shaking hands). These glad hands have rubber seals to form a tight connection and prevent air from escaping. Make sure that the glad hands are locked in place and free of damage or air leaks.

Not quite. The answer is a supply tank.

An Integrated Solution Air Dryer (AD-IS®) and Reservoir System provides an integrated air dryer, purge reservoir, governor, and a number of the charging valve components in a module. The air dryer is so efficient that a supply ("wet") tank is not needed. Instead, the AD-IS sends air directly to the primary and secondary service ("dry") tanks.

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 spring brakes.

In some vehicles, a control handle on the dashboard may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.

Not quite. The answer is 32 feet

55 mph = 81 feet per second. Air brake lag typically lasts about 0.4 seconds. Therefore, the lag distance equals 81 feet per second × 0.4 seconds = 32 feet.

Not quite. The answer is 2 psi per minute.

Perform the first test of air leakage with the engine off and the brakes released. The air-loss rate should be less than 3 psi per minute for a combination vehicle and less than 2 psi per minute for a bus or straight truck. After you've tested this initial drop, now test air leakage with the engine off and the foot brake fully applied. This time, the air-loss rate should be less than 4 psi per minute for a combination vehicle and less than 3 psi per minute for a bus or straight truck.

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 Safety valve

Drum brakes are on each of your vehicle's axles and contain about 10 different parts, including the axle, slack adjuster, and brake drum itself. While a safety valve is part of the air brake system, it is not part of the drum brake.