Massachusetts CDL: Air Brakes Practice Test

Not quite. The answer is brake in a straight line.

In controlled braking, you brake as hard as you can without locking the wheels. As you do this, keep your vehicle going relatively straight.

Not quite. The answer is 1 inch

The slack adjusters won't move if the parking brakes are on. To check the slack adjusters, park on level ground, chock the wheels, and then release the parking brakes. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the pushrod attaches to it, it probably needs adjustment.

Not quite. The answer is depress 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. (Clearly, 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 you have to press the brake pedal harder than usual to control your speed.

If your brakes begin to fade, you'll notice that you have to depress the brake pedal harder to achieve the same braking effect as before. If your vehicle is so equipped, check the application pressure gauge to see just how much pedal effort you are employing. If the brakes continue to fade, you may not be able to slow down or stop at all.

Not quite. The answer is Clockwise until the bolt stops turning

Adjusting an automatic slack adjuster is an emergency procedure that you should perform only if your brakes have failed on the road. Most automatic slack adjusters have a hexagonal (six-sided) adjusting bolt. For this type of slack adjuster, first turn the bolt clockwise until it stops turning. If the slack adjuster has a square adjusting bolt, first turn the bolt counterclockwise until it stops turning. Then with either type of bolt, back it off about half a turn in the opposite direction.

Not quite. The answer is the emergency brake system.

The system that uses parts of the service and parking brake systems to stop the vehicle in a brake system failure is called the emergency brake system.

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 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 They help maintain correct pushrod stroke.

An automatic slack adjuster automatically compensates for normal brake lining wear by adjusting pushrod stroke. Automatic slack adjusters should not normally require manual adjustment of pushrod stroke. However, they still require lubrication at regular intervals.

Not quite. The answer is reduce air pressure at the front wheels.

Most modern tractors are equipped with an automatic front axle ratio valve, which supersedes the manual front brake limiting valve in older tractors. In light-to-moderate braking, the ratio valve automatically reduces air pressure (and hence braking force) to the front wheels to reduce the chance of front-wheel lockup (which could result in a skid) on slippery roads.

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 activates when the wheels are about to lock up.

ABS does not have much effect on your everyday driving. It does not affect your braking capability or shorten your stopping distance. It just helps you keep control of your vehicle by activating when your wheels are about to lock up.

Not quite. The answer is To avoid injury from high-pressure air and debris

Compressed air may be exhausted from several different ports in the air braking system. This air travels at a very high speed from the vehicle and carries moisture, oil, dirt, and dust. It can be dangerous if it hits your eyes or ears. Know the location of all air exhaust ports so you can keep away from them, such as when you're draining the air tanks.

Not quite. The answer is Typically, one brake system operates the brakes on the front axle, and the other system operates the brakes on the rear axles.

A dual air brake system has two separate air brake systems, each with its own air tanks, hoses, lines, etc. Both systems are controlled by the one brake pedal. Typically, the secondary system operates the brakes on the front axle (and possibly one rear axle as well), and the primary system operates the brakes on the rear axles.

Not quite. The answer is All of the above are correct.

On both tractor-trailers and single vehicles, to test whether the spring brakes will come on automatically, use the same method you use for testing the low air pressure warning signal: Step on and off the brake pedal, this time until you reach an even lower psi reading and the parking brake valve closes or pops out.

Not quite. The answer is about 250 °C.

Brakes are designed to withstand quite a lot of heat. The brake components can withstand temperatures up to about 250° C (482 °F) under hard braking.

Not quite. The answer is If the brakes are very hot

If the brakes are too hot, avoid using the parking brakes because the heat may warp and damage the brakes. Also avoid using the parking brakes if they are wet and at risk of freezing. If the parking brakes freeze, your vehicle will be immobilized. In both of these situations, use wheel chocks instead.

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 To show how much pressure is in the air tanks.

The air supply pressure gauge is connected to the air tank (or possibly to both tanks in a dual air tank system). It shows how much air pressure is in the air tanks.

Not quite. The answer is 150 psi

The safety valve, which protects the system from excessive pressure, is usually set to open when the system reaches 150 psi.