CDL Air Brakes Test Answers
CDL air brakes answers for independent study: review 59 air brakes knowledge test questions, the ones test takers report, each answered from and quoted from Section 5 (Air Brakes) of the California Commercial Driver Handbook, the national model text. Not an official answer key for any state's exam.
Study questions for the air brakes knowledge test of the commercial driver license (CDL), answered from Section 5 (Air Brakes) of a state CDL manual (national model text): the California Commercial Driver Handbook (DL 650, rev. 12/2019) from California DMV. Covers the parts of an air brake system (compressor, governor, air tanks and drains, alcohol evaporator, safety valve, gauges, warning signals and spring brakes), dual air brake systems, slack adjusters, the in-cab air brake checks and their leakage limits, brake lag and stopping distance, brake fade, downgrade braking, emergency stops and parking. Every answer is grounded in the handbook text.
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01In an air brake system, what does the air compressor governor control?
When the air compressor pumps air into the air storage tanks. The governor stops the compressor at the cut-out level (around 125 psi) and lets it start pumping again at the cut-in level (around 100 psi).
VERIFIED AGAINST THE SOURCE
“When the tank pressure falls to the cut-in pressure (around 100 psi), the governor allows the compressor to start pumping again.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗02For an air brake system in good condition, the governor lets the air compressor start pumping again at the cut-in level, which is about how many psi?
About 100 psi. When tank pressure falls to the cut-in pressure (around 100 psi), the governor lets the compressor start pumping again; it stops at the cut-out level (around 125 psi).
VERIFIED AGAINST THE SOURCE
“The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the cut-out level (around 125 pounds psi), the governor stops the compressor from pumping air. When the tank pressure falls to the cut-in pressure (around 100 psi), the governor allows the compressor to start pumping again.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗03How is the air compressor connected to the engine?
Through gears or a v-belt. Some answer lists say gears and hoses; the handbook says the compressor is connected to the engine through gears or a v-belt.
VERIFIED AGAINST THE SOURCE
“The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a v-belt.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗04If the air compressor on your truck has its own oil supply, when should you check its oil level?
Before driving. The compressor may have its own oil supply or be lubricated by engine oil; if it has its own supply, check the oil level before driving.
VERIFIED AGAINST THE SOURCE
“It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗05Where is the compressed air in an air brake system stored?
In the air storage tanks (reservoirs). The tanks hold enough air to let the brakes be used several times even if the compressor stops working.
VERIFIED AGAINST THE SOURCE
“Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗06Why should you drain the water from compressed air tanks?
Compressed air usually has some water and compressor oil in it, which collect in the bottom of the tank. The water can freeze in cold weather and cause brake failure.
VERIFIED AGAINST THE SOURCE
“Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. The water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗07If you do not have automatic tank drains, how often should you drain the oil and water from the air storage tanks?
At the end of each day of driving. Manual drains must be drained by you at the end of each working day to remove moisture and oil; otherwise the brakes could fail.
VERIFIED AGAINST THE SOURCE
“If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗08If your air brake system has an alcohol evaporator, what is it there to do?
Reduce the risk of ice in the air brake valves and other parts during cold weather. It puts alcohol into the air system.
VERIFIED AGAINST THE SOURCE
“Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗09If your vehicle has an alcohol evaporator, what should you do every day during cold weather?
Check the alcohol container and fill it up as necessary. Daily air tank draining is still needed to get rid of water and oil unless the system has automatic drain valves.
VERIFIED AGAINST THE SOURCE
“Check the alcohol container and fill up as necessary (every day during cold weather). Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗10Some air brake systems have an alcohol evaporator. What may happen if the unit is not kept filled with alcohol?
Ice can form in the air brake valves and other parts in cold weather, and ice inside the system can make the brakes stop working.
VERIFIED AGAINST THE SOURCE
“This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗11The safety valve in the first air tank is usually set to open at how many psi?
150 psi. The safety valve protects the tank and the rest of the system from too much pressure. If it releases air, something is wrong and a mechanic should fix it.
VERIFIED AGAINST THE SOURCE
“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.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗12What does the brake pedal do in an air brake system?
It controls the air pressure applied to put on the brakes. Pushing the pedal down harder applies more air pressure; letting up reduces the air pressure and releases the brakes.
VERIFIED AGAINST THE SOURCE
“You engage the brakes by pushing down the brake pedal (also called a foot valve or treadle valve). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗13Repeatedly pressing and releasing the brake pedal (fanning) without need may result in what?
Loss of air pressure. Fanning can let air out faster than the compressor can replace it, and if the pressure gets too low the brakes will not work.
VERIFIED AGAINST THE SOURCE
“Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗14What is the most common type of foundation brake on heavy vehicles?
The S-cam drum brake. Foundation brakes are used at each wheel.
VERIFIED AGAINST THE SOURCE
“Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. The parts of the brake are discussed below.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗15When you apply the brakes, the brake shoes and linings are pushed against what?
The inside of the brake drum. This causes friction, which slows the vehicle and creates heat.
VERIFIED AGAINST THE SOURCE
“To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat).”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗16What are the slack adjusters on S-cam brakes?
The levers that link each brake chamber's push rod to the brake camshaft. When you brake, air pushes the rod out, moving the slack adjuster and twisting the camshaft, which turns the S-cam and presses the shoes against the drum. One that moves more than about one inch where the push rod attaches probably needs adjustment.
VERIFIED AGAINST THE SOURCE
“Check slack adjusters on S-cam brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes so you can move the slack adjusters. 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 push rod attaches to it, it probably needs adjustment.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗17How should you check the free play of the manual slack adjusters on S-cam brakes, and where should you park to do it?
Park on level ground and chock the wheels, release the parking brakes, then wear gloves and pull hard on each slack adjuster you can reach. If one moves more than about one inch where the push rod attaches, it probably needs adjustment.
VERIFIED AGAINST THE SOURCE
“Check slack adjusters on S-cam brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes so you can move the slack adjusters. 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 push rod attaches to it, it probably needs adjustment.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗18What does the air supply pressure gauge show the driver?
How much pressure is in the air tanks. A dual air brake system has a gauge for each half of the system, or a single gauge with 2 needles.
VERIFIED AGAINST THE SOURCE
“All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with 2 needles). Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗19The application pressure gauge shows how much air pressure you are applying to what?
The brakes. Not every vehicle has this gauge.
VERIFIED AGAINST THE SOURCE
“This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗20How can you tell that your brakes are fading?
You have to push harder on the brake pedal to control your speed on a downgrade. Increasing application pressure to hold the same speed means the brakes are fading; slow down and use a lower gear.
VERIFIED AGAINST THE SOURCE
“Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction, and cause expansion of the brake drums.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗21All vehicles equipped with air brakes must have what gauge?
A supply pressure gauge connected to the air tank, which shows how much pressure is available in the air tanks. A low air pressure warning signal is also required.
VERIFIED AGAINST THE SOURCE
“A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on when the air pressure in the tanks falls between 55 and 75 psi (or 1/2 the compressor governor cutout pressure on older vehicles).”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗22What does the stop light switch on an air brake vehicle do?
It is an electric switch, worked by air pressure, that turns on the brake lights when you put on the air brakes so drivers behind you are warned.
VERIFIED AGAINST THE SOURCE
“Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗23True or false: front wheel brakes are good under all conditions.
True. Front wheel braking is good under all conditions; tests have shown front wheel skids from braking are not likely even on ice.
VERIFIED AGAINST THE SOURCE
“Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗24The parking or emergency brakes of trucks and buses can be legally held on by what?
Spring pressure (mechanical force), because air pressure can eventually leak away. Some answer lists say air pressure; the handbook says these brakes must be held on by mechanical force, and spring brakes are usually used.
VERIFIED AGAINST THE SOURCE
“Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving. If the air pressure is removed, the springs put on the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗25During normal driving, what usually holds the spring brakes back?
Air pressure. Powerful springs are held back by air pressure while driving; if the air pressure is removed, the springs put on the brakes. One answer list says spring pressure; the handbook says air pressure.
VERIFIED AGAINST THE SOURCE
“Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving. If the air pressure is removed, the springs put on the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗26What are spring brakes on an air brake vehicle?
Brakes that use powerful springs, held back by air pressure while driving, to serve as the parking and emergency brakes. If the air pressure is removed, the springs put on the brakes.
VERIFIED AGAINST THE SOURCE
“Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving. If the air pressure is removed, the springs put on the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗27What does the braking power of the spring brakes depend on?
The brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
VERIFIED AGAINST THE SOURCE
“The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗28If the spring brakes are on, when should you push the brake pedal down?
Never. The brakes could be damaged by the combined forces of the springs and the air pressure.
VERIFIED AGAINST THE SOURCE
“Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗29What does a modulating control valve let you control?
The spring brakes. A spring-loaded control handle on the dash lets you apply the spring brakes gradually, so you can control them if the service brakes fail.
VERIFIED AGAINST THE SOURCE
“In some vehicles a control handle on the dash board 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.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗30If your truck or bus has dual parking control valves, what can you use the pressure from the separate air tank to do?
Release the spring brakes so you can move the vehicle in an emergency. There is only enough air in the separate tank to do this a few times.
VERIFIED AGAINST THE SOURCE
“When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗31Which brake system applies and releases the brakes when the driver uses the brake pedal during normal driving?
The service brake system.
VERIFIED AGAINST THE SOURCE
“Air brakes are really 3 different braking systems: service brake, parking brake, and emergency brake. The: Service brake system applies and releases the brakes when you use the brake pedal during normal driving.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗32Air brakes combine three different braking systems. They are the service brakes, the parking brakes and the ___ brakes.
Emergency brakes. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
VERIFIED AGAINST THE SOURCE
“Air brakes are really 3 different braking systems: service brake, parking brake, and emergency brake. The: Service brake system applies and releases the brakes when you use the brake pedal during normal driving.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗33What is a dual air brake system on a truck or bus?
Two separate air brake systems that use a single set of brake controls. Each system has its own air tanks, hoses and lines; one usually works the rear axle brakes and the other the front axle brakes.
VERIFIED AGAINST THE SOURCE
“A dual air brake system has 2 separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗34Before driving a truck or bus with a dual air brake system, the air pressure in each system should be built up to at least how many psi?
100 psi, in both the primary and secondary systems.
VERIFIED AGAINST THE SOURCE
“Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗35Your truck or bus has a dual air brake system. If the low air pressure warning comes on for only one of the systems while driving, what should you do?
Stop right away and safely park the vehicle, then have the air brake system fixed. The warning should come on before pressure drops below 55 psi in either system.
VERIFIED AGAINST THE SOURCE
“The warning light and buzzer should come on before the air pressure drops below 55 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗36If your truck has a dual air system and one of the systems is very low on pressure, what happens?
Either the front or the rear brakes will not be operating fully, so it will take you longer to stop.
VERIFIED AGAINST THE SOURCE
“If one air system is very low on pressure, either the front or rear brakes will not be operating fully. This means it will take you longer to stop.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗37With a properly working dual air brake system, how quickly should air pressure build from 85 to 100 psi?
Within 45 seconds, with the engine at normal operating idle.
VERIFIED AGAINST THE SOURCE
“Observe the air gauge to determine if the pressure builds at the proper rate. For dual air systems, the pressure should build from approximately 85 to 100 psi within 45 seconds.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗38With a single air brake system (on vehicles built before 1975), how should the air pressure build?
From about 50 to 90 psi within 3 minutes, with the engine at normal operating idle.
VERIFIED AGAINST THE SOURCE
“For single air systems (in pre-1975 vehicles), the pressure should build from approximately 50 to 90 psi within 3 minutes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗39If the air compressor develops a leak, what keeps the air in the tanks?
The one-way check valve between the air compressor and the first reservoir. It lets air flow in one direction only.
VERIFIED AGAINST THE SOURCE
“This device allows air to flow in one direction only. All air tanks on air-brake vehicles must have a check valve located between the air compressor and the first reservoir (CVC 26507). The check valve keeps air from going out if the air compressor develops a leak.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗40What is the first thing you should do when the low air pressure warning comes on?
Stop and safely park the vehicle as soon as possible. There might be an air leak, and controlled braking is possible only while enough air remains in the tanks.
VERIFIED AGAINST THE SOURCE
“If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗41How can you test the low air pressure warning signal?
With enough air pressure that the warning is off, turn the key to on (the engine may be on or off) and fan off the air pressure by rapidly applying and releasing the brake pedal. The warning must come on before pressure drops below 55 psi or the level the manufacturer specifies.
VERIFIED AGAINST THE SOURCE
“To perform this test the vehicle must have enough air pressure so the low-pressure warning signal is off. The engine maybe on or off; however, the key must be in the on or battery charge position. Next, the driver begins fanning off the air pressure by rapidly applying and releasing the foot brake. Low-air warning devices (buzzer, light, and flag) must activate before air pressure drops below 55 psi or the level specified by the manufacturer.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗42How can you check that the spring brakes come on automatically when air pressure in the system drops?
With the parking brake released, keep fanning off the air pressure with the brake pedal. Normally between 20 and 45 psi (or the manufacturer's level) the parking brake valve should close (pop out). One answer list says to apply the parking brake first; the handbook says it must be released.
VERIFIED AGAINST THE SOURCE
“To perform this test, the parking brake (all vehicles) and tractor protection valve (combination vehicles) must be released; (engine running or not) as the driver fans off the air pressure. Normally between 20-45 psi (or the level specified by the manufacturer), on a tractor-trailer combination vehicle, the tractor protection valve and parking brake valve should close (pop out).”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗43With the engine off and the brakes applied, the air loss rate for a straight truck or bus should not be more than how much?
3 psi in one minute (4 psi for a combination vehicle). This is the applied leakage test, held for one minute after the pressure is built to governor cut-out.
VERIFIED AGAINST THE SOURCE
“The driver checks the air gauge to see that the air pressure drops no more than 3 pounds in one minute (single vehicle) or 4 pounds in 1 minute (combination vehicle) and listens for air leaks.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗44With the engine off and the brakes released, a straight truck or bus air brake system should not leak more than how many psi per minute?
2 psi per minute. With the engine off and all brakes released, the limit is 2 psi for single vehicles, 3 psi for a combination of 2 vehicles and 5 psi for 3 or more. One answer list says 3; the handbook gives 3 psi only for a 2-vehicle combination or for the applied test.
VERIFIED AGAINST THE SOURCE
“With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for 1 minute. The air pressure should not drop more than: 2 psi for single vehicles. 3 psi for a combination of 2 vehicles. 5 psi for a combination of 3 or more vehicles.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗45How should you test the air leakage rate with the brakes released?
With a fully charged system, turn off the engine, release all brakes, let the system settle and time for 1 minute. The pressure should not drop more than 2 psi for a single vehicle.
VERIFIED AGAINST THE SOURCE
“With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for 1 minute. The air pressure should not drop more than: 2 psi for single vehicles. 3 psi for a combination of 2 vehicles. 5 psi for a combination of 3 or more vehicles.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗46How should you check that your service brakes are working properly?
Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph) and apply the brakes firmly with the brake pedal. Note any pulling to one side, unusual feel or delayed stopping.
VERIFIED AGAINST THE SOURCE
“Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Note any vehicle pulling to one side, unusual feel, or delayed stopping action.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗47If you must make an emergency stop in an air brake vehicle, you should brake so that you:
Keep the vehicle in a straight line and can still turn if it becomes necessary, using the controlled braking or stab braking method.
VERIFIED AGAINST THE SOURCE
“You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the controlled braking or stab braking method.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗48What is emergency controlled braking?
Applying the brakes as hard as you can without locking the wheels, while keeping steering wheel movements very small.
VERIFIED AGAINST THE SOURCE
“Controlled Braking. With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗49What is emergency stab braking?
Applying the brakes all the way, releasing them when the wheels lock up, and applying them fully again as soon as the wheels start rolling.
VERIFIED AGAINST THE SOURCE
“Stab Braking. Apply your brakes all the way. Release the brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗50Why does air braking take more time than hydraulic braking?
Because it takes a little time (1/2 second or more) for the air to flow through the lines to the brakes. This delay is called brake lag.
VERIFIED AGAINST THE SOURCE
“This is the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (1/2 second or more) for the air to flow through the lines to the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗51What makes the total stopping distance for air brakes longer than for hydraulic brakes?
Brake lag distance, which is added to perception, reaction and braking distance.
VERIFIED AGAINST THE SOURCE
“Thus, the total stopping distance for vehicles with air brake systems is made up of 4 different factors. Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance = Total Stopping Distance”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗52At 55 mph on dry pavement, the air brake lag distance adds about how many feet to your stopping distance?
About 32 feet.
VERIFIED AGAINST THE SOURCE
“The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 450 feet.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗53Under ideal conditions, a truck with air brakes going 55 mph would require a stopping distance of about how many feet?
Over 450 feet for an average driver, including about 32 feet of brake lag distance.
VERIFIED AGAINST THE SOURCE
“The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 450 feet.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗54Excessive use of the service brakes results in overheating, which can lead to what?
Brake fade. The heat causes chemical changes in the brake lining that reduce friction, and it causes the brake drums to expand.
VERIFIED AGAINST THE SOURCE
“Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction, and cause expansion of the brake drums.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗55True or false: on a long or steep downgrade, using the brakes is only a supplement to the braking effect of the engine.
True. Get into the correct low gear first; the brakes only supplement the engine braking effect.
VERIFIED AGAINST THE SOURCE
“Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗56Going down a long, steep grade in the proper gear, you should brake until your speed is about how many mph below your safe speed?
About 5 mph below your safe speed, then release the brakes. The application should last about 3 seconds.
VERIFIED AGAINST THE SOURCE
“When your speed has been reduced to approximately 5 mph below your safe speed, release the brakes. (This application should last for about 3 seconds.)”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗57You are going down a steep downgrade and have reached your safe speed of 40 mph. You apply the service brakes until your speed drops to how many mph?
35 mph, then release the brakes. Repeat as often as necessary until the end of the downgrade.
VERIFIED AGAINST THE SOURCE
“If your safe speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗58When should the parking brakes be used on an air brake vehicle?
Any time you park, except when the brakes are very hot or very wet in freezing temperatures. Never leave the vehicle unattended without applying the parking brakes or chocking the wheels.
VERIFIED AGAINST THE SOURCE
“Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗59Which of these is NOT a proper time to use the parking brakes on an air brake vehicle?
When the brakes are very hot (for example, just after coming down a steep grade), since heat can damage them. They also should not be used when the brakes are very wet in freezing temperatures.
VERIFIED AGAINST THE SOURCE
“Do not use the parking brakes if the brakes are very hot (from just having come down a steep grade), or if the brakes are very wet in freezing temperatures. If they are used while they are very hot, they can be damaged by the heat.”
— California DMV, California Commercial Driver Handbook (DL 650, rev. 12/2019), Section 5: Air Brakes ↗Know questions we're missing?
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