Air brake systems are three braking systems combined. What are these braking systems?
The air brake system uses compressed air to combine three different brake systems. It combines the service brake system, the parking brake system, and the emergency brake system.
The air compressor is an important part of the air brake system. What is the purpose of the air compressor?
The air compressor’s primary purpose is pumping air into the storage tanks so that it can be used by the air brake system.
The emergency brake system uses parts of what brake systems to stop the vehicle in case of a brake system failure?
If the brakes fail, the emergency brake can use parts of these two systems to assist in stopping the vehicle.
The cut-out level and cut-in level for the air compressor governor are roughly what pressure per square inch (PSI)?
The governor determines when the air compressor will stop and start. When the pressure per square inch (PSI) reaches roughly 125 PSI, the governor stops the compressor. When the pressure falls below roughly 100 PSI, the governor starts the compressor again.
Compressed air usually contains what substances that are harmful to the brake system?
Water and some compressor oil are harmful to the brake system because the water can freeze and therefore cause brake failure.
For a manual air tank, the water and oil are expelled by _____.
Manual air tanks have a drain valve that must be turned a quarter turn in order to drain the built-up water and oil from the tank.
Some air brake systems have an alcohol evaporator. What is the purpose of the alcohol evaporator?
The alcohol evaporator puts alcohol into the system. This alcohol helps keep ice from forming in the system, which can cause the brakes to fail.
What device protects the tank and the rest of the air brake system from too much pressure?
The safety valve is set to automatically open at a certain PSI, typically about 150 PSI. This is to ensure the tank does not build up too much pressure, which can be dangerous.
How does letting up on the brake pedal release the brakes?
When a person lets go of the brake pedal, the air pressure in the air tanks is reduced by letting air out of the system. This causes the brakes to be released.
What can be caused by the unnecessary pressing and releasing of the brake pedal?
If the brake pedal is pressed and released unnecessarily, it can cause air to be released faster than the compressor can replace it. This can cause the brakes to not work because of low pressure in the air tank.
When you push the brake pedal down, what force or forces push back against your foot?
When you push on the brake pedal, two forces will resist your foot. These two forces are the spring in the physical pedal and the air pressure coming from the air brake system.
What is the most common type of foundation brakes?
There are several types of foundation brakes, but the most common type is the S-cam drum brakes.
When the brakes are applied, brake shoes and linings are pushed against what?
In order to stop, the brakes must be applied. This causes the brake shoes and linings to to be pushed against the inside of the drum and create friction. This friction slows the vehicle down.
What are forced away from each other and against the inside of the brake drum by the S-cam?
The twist of the brake cam shaft turns the S-cam. This causes the brake shoes to be pushed away from each other and against the inside of the brake drum.
Wedge brakes have a chamber push rod. What does the push rod do?
There is a chamber push rod that wedges between the two brake shoes. It pushes them apart and against the inside of the brake drum causing friction.
How do disc brakes primarily differ from S-cam brakes?
Air pressure is used the same way in both braking systems, but instead of turning an S-cam, a power screw is turned, clamping a disc between the brake lining pads of a caliper
Which air braking system is the most common?
Of the three air braking systems, S-cam brakes are used the most.
What does the supply pressure gauge indicate?
The supply pressure gauge is used for measuring the amount of pressure left in the air tanks.
If application pressure must be increased to maintain a certain speed while going down a steep grade, then this most likely means what?
When application pressure must be increased to maintain a consistent speed, it most likely means the brakes are fading and one must shift to a lower gear to slow the vehicle down.
A low air pressure warning comes on before what PSI?
A low air pressure warning, such as a red light and/or a buzzer, must come on before the air tank falls below 60 PSI (or 1/2 the compressor governor cut-out pressure on some older vehicles).
What does the front brake limiting valve in some older vehicles do when turned to “slippery”?
There are both “normal” and “slippery” settings. When set on “slippery,” the standard air pressure sent to the front brakes is cut in half. This was believed to help maintain control of the vehicle in slippery conditions.
Why are front brake limiting valves not present in newer trucks?
Tests have shown that a front brake limiting valve is not helpful, even in icy conditions. It reduces the stopping power of the vehicle and the likelihood of a front wheel skid from braking has been shown to be very rare
What does it mean when the “wig wag” drops a mechanical arm into your view?
The “wig wag” is another less common type of warning when the PSI in the air tanks drops below 60. Usually the warning is a red light or a buzzer. The “wig wag” drops a mechanical arm into view until the pressure in the air tanks has been restored to normal.
What can a leak in the air brake system cause?
When the air brake system has a leak, the air is let out of the system. This air is what holds the springs back, which means the spring brakes will be applied when the air is released.
When the parking brake is engaged, what does it do to the spring brakes?
When a parking brake is engaged, it enables the driver to let the air out of the spring brakes.
What should be done when the spring brake low air pressure buzzer and warning light first come on?
As soon as the warning light and buzzer come on, it is essential to safely stop the vehicle as it is not guaranteed that the emergency braking system will be able to stop the vehicle.
Some vehicles have a modulating control valve. What does a modulating control valve do?
The modulating control valve is a control handle on the dashboard that enables you to gradually apply the spring brakes. The more you turn the valve, the harder the brakes are applied
What controls the parking brakes in newer vehicles?
The diamond-shaped, yellow, push-pull control knob is the control for the parking brakes in newer vehicles. It must be pulled out to put the parking brakes on and pushed in to release them
What should you never do when the spring brakes are on?
If the brake pedal is pushed while the spring brakes are on, it can damage the brakes by combining the forces of the springs and the air pressure.
Some vehicles, such as buses, have a separate air tank used to do what?
Some vehicles have a separate air tank used to release the spring brakes in the case of an emergency. This allows the vehicle to be moved. Careful planning must be used, though, as there is only enough air to move it a short ways.
Most heavy-duty vehicles nowadays have a dual air brake system. What is a dual air brake system?
Dual air brake systems are used by most new heavy-duty vehicles to increase safety. They use two separate air brake systems with a single set of brake controls.
In a dual air brake system, which parts of braking does each system typically operate?
There is a primary and secondary system. The primary usually operates the regular brakes on the rear axle/axles and the secondary usually operates the regular brakes on the front axle (and possibly a rear axle). Both systems send air to the trailer brakes.
Before driving a vehicle with a dual air system, the driver must allow what to happen?
Both air brake systems must be fully functional before the driver should use the vehicle. The driver must allow both systems to have at least 100 PSI so that the brake systems work properly.
Checking the air compressor drive belt should be done during what step of the basic seven-step inspection procedure?
The condition and tightness of the belt should be checked when checking the other belts. This falls under step 2-engine compartment checks.
Checking the manual slack adjusters on the S-cam brakes should be done during what step of the basic, seven-step inspection procedure?
Checking the slack adjusters should be done while the vehicle is parked on level ground and the wheels are chocked. This falls under step 5-the walkaround inspection.
Which of these should not be done during the final step of the basic, seven-step inspection procedure?
Checking the brake drums (or discs), linings, and hoses should be done during step 5-the walkaround inspection.
Chocking the wheels, releasing the parking brake when there is enough pressure to do so, shutting the engine off, and then stepping on and off the brake pedal will test what?
Doing this should cause the parking brake knob to pop out when the air pressure falls to the manufacturer’s specification (usually 20PSI to 40 PSI). This will cause the spring brakes to come on.
With the engine at operating RPM, air tank pressure in a dual system should build from 85 PSI to 100 PSI within how much time?
In a vehicle with standard air tanks, the time it takes to increase from 85 PSI to 100 PSI should be about 45 seconds. If the pressure does not build fast enough, the pressure may become too low while driving, which can be dangerous.
To test the air leakage rate, one must turn off the engine (with a fully charged air system), turn the key to the “on” position, release the service brake, and do what?
Timing the air pressure drop can determine if the vehicle is losing too much air pressure too quickly. This can cause loss of brakes while driving.
When testing the air leakage rate, the initial loss rate should be less than what PSI/minute for single vehicles and what PSI/minute for combination vehicles?
The loss rate for single vehicles should be less than 2 PSI and the loss rate for combination vehicles should less than 3 PSI in 1 minute. After applying 90 PSI or more with the brake pedal, if the loss rate exceeds 3 PSI for single vehicles and 4 PSI for combination vehicles, the vehicle should be inspected for air leaks.
Applying the brakes as hard as you can without locking the wheels is called what?
When a person presses down on the brake pedal as hard as they can without locking the wheels, it is called controlled braking.
Pressing on the brakes as hard as you can, releasing when they lock up, then pressing the brakes fully again when the wheels are rolling is called what?
When a person presses as hard as they can on the brakes, releases the brakes when they lock up, then press as hard as they can again on the brakes when the wheels have begun rolling, it is called stab braking.
With air brakes, there is an added delay when stopping. What causes this?
With hydraulic brakes, the brakes work instantly. But with air brakes, there is about a 0.5 second delay while the air flows through the lines to the brakes before the brakes begin working.
The total stopping distance for vehicles with air brake systems is made up of four factors. Which of the following is not one of these factors?
The correct formula for stopping is the following: Perception distance + Reaction distance + Brake lag distance + Effective braking distance = Total stopping distance
What is the correct way to slow a large vehicle down on long downgrades?
Brakes can heat up if used excessively. When they become too hot, such as when used on long downgrades, they will fade. They can fade to the point where they will not slow the vehicle down. Shifting into a lower gear and driving at a slow enough speed that a small amount of braking will keep you from speeding up is the best method of avoiding this.
Why is it especially important to have your brakes adjusted before going down steep grades?
When the brakes have not been properly balanced, it will cause some brakes to do more work than others. This will heat up these brakes and they will lose their stopping power.
Chemical changes in the brake lining, which reduce friction and cause expansion of the brake drums, is known as what?
Excessive heat causes chemical changes to the brake lining, which reduces friction, and it causes expansion of the brake drums. As this happens, the shoes and linings have to move farther to contact the drums, causing the brakes to fade.
If your “safe” speed is 45 miles per hour and you are traveling at 60 miles per hour on a downgrade, you should apply the brakes until you have slowed to what speed?
When on a downgrade, you should apply the brakes until you are traveling at 5 miles below your “safe” speed.
If your “safe” speed is 45 miles per hour, you should not apply the brakes until you have reached what speed?
You should not apply the brakes until you have reached your “safe” speed. When you do reach your safe speed you should apply the brakes gradually until you reach 5 MPH below your safe speed.
In what instances should you not use the parking brakes when parking?
Parking brakes can be damaged if used while they are very hot. If they are used while wet in freezing conditions, they can freeze, causing you to not be able to move your vehicle.
If parking brakes are not being used when the vehicle is parked, what must be done instead?
At times the parking brakes should not be used because they are too hot or wet during freezing temperatures. At this time, wheel chocks should be used instead while the brakes cool or dry out.
If the brakes are wet, what can be done to dry them out?
If the brakes are wet, one should use them while driving in a low gear. This will heat them up and dry them out.