California CDL: Air Brakes Practice Test

Not quite. The answer is 1/4 inch

According to federal regulations on air brakes, the brake linings of non-steering-axle brakes must be at least 1/8 inch thick for disc brakes or at least 1/4 inch thick for drum brakes. The brake linings of steering-axle drum brakes can be thinner.

Not quite. The answer is 35 mph.

On a steep downgrade, continuous application of the service brakes might cause them to fade (become less effective). Instead, apply your brakes sparingly as a supplement to engine braking. Select a "safe" speed. Let your vehicle reach this speed. Brake gently until you are going 5 mph slower. (In this case, with a "safe" speed of 40 mph, you should brake gently until your speed drops to 35 mph.) Then let your speed increase back to the "safe" speed. Keep varying your speed up and down this way. This technique is known as snub braking or snubbing.

Not quite. The answer is Never

Applying the spring brakes and the air brakes at the same time may put too much force on the brakes and can actually damage them.

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 With the engine off, step on and off the brake pedal to lower the air pressure below 55 psi.

The best way to test the low air pressure warning signal is to release the parking brake (all vehicles), release the tractor protection valve (on combination vehicles), and turn off the engine of your vehicle while leaving the electrical power on. Then step on and off the brake pedal. The low air pressure warning signal should come on when the pressure falls below 55 psi. You should never need to get down to 30 psi.

Not quite. The answer is between 55 and 75 psi.

A warning signal you can see must come on when the air pressure in the tanks falls to between 55 and 75 psi (or one-half the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on.

Source: CVC § 26506

Not quite. The answer is slack adjuster; brake camshaft

To apply the S-cam drum brakes, air is let into each brake chamber. Air pressure pushes the pushrod out, moving the slack adjuster, thus rotating the brake camshaft. As the S-cam on the camshaft rotates, it forces the brake shoes against the inside of the brake drum.

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 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 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 braking effect of the engine.

Going down a hill, you should keep your engine in a low gear to keep your speed low, and use the air brakes as a supplemental tool to slow down.

Not quite. The answer is to tell you how much air is in the tank.

The supply pressure gauge tells you how much air is in the tank at all times. In some vehicles, it may be connected to a warning bell or light.

Not quite. The answer is bring the vehicle to a safe stop right away.

If air pressure drops to 20–45 psi, the spring brakes will come fully on. Don't let things get that far. If the low air pressure warning light and buzzer come on, bring the vehicle to a safe stop right away, while you can still control the brakes.

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 actual stopping distance. It just helps you keep your vehicle in control by activating when your wheels are about to lock up.

Not quite. The answer is Tap on the brake drums.

If a brake lining has gotten frozen to the brake drum, you should be able to free it by using a hammer to tap on the drum near the brake lining. Be careful not to hit the brake lining because you might damage it. Note that you may also need to free the other brake lining in that drum brake. If the valves or other parts of the air braking system are frozen, you may be able to get them unstuck by putting commercial air brake antifreeze (NOT pure methyl alcohol) into the trailer air supply (red) line. However, this will not free frozen brake linings.

Not quite. The answer is you will still have enough air in the storage tanks to apply the air brakes several times.

The air storage tanks hold enough air for you to apply the air brakes several times.

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 5 mph slower.

On a steep downgrade, don't keep applying your brakes continuously because this might cause them to fade (become less effective). Instead, apply your brakes sparingly as a supplement to engine braking. Select a "safe" speed. Let your vehicle reach this speed. Brake gently until you are going 5 mph slower. Then let your speed increase back to the "safe" speed. Keep varying your speed up and down this way. This technique is known as snub braking or snubbing.

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 Use gloves and pull hard on each slack adjuster you can reach.

To check the slack adjusters, try to pull on them manually while wearing gloves. If there is an inch or more of "give," then there is an issue and they should be fixed.