Air brake parts and dual systems
Questions and answers
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Air brakes are really how many different braking systems?
Three: service, parking, and emergency
An air brake setup is made of the service brake, the parking brake, and the emergency brake systems.
Section 5 · Air Brakes ↗PDF, 41 MB -
Which air brake system applies and releases the brakes when you use the brake pedal in normal driving?
The service brake system
The service brake system is the one worked by the pedal during normal driving.
Section 5 · Air Brakes ↗PDF, 41 MB -
What does the emergency brake system use to stop the vehicle in a brake system failure?
Parts of the service and parking brake systems
The emergency brake system borrows parts of the service and parking systems.
Section 5 · Air Brakes ↗PDF, 41 MB -
What does the air compressor do?
Pumps air into the air storage tanks
The compressor fills the storage tanks, also called reservoirs.
Section 5 · 5.1.1 – Air Compressor ↗PDF, 41 MB -
How is the air compressor connected to the engine?
Through gears or a v-belt
The compressor is driven by the engine through gears or a v-belt.
Section 5 · 5.1.1 – Air Compressor ↗PDF, 41 MB -
If the air compressor has its own oil supply, what should you do before driving?
Check the oil level
A compressor with its own oil supply needs its oil level checked before you drive.
Section 5 · 5.1.1 – Air Compressor ↗PDF, 41 MB -
How may the air compressor be cooled?
By air or by the engine cooling system
The compressor may be air cooled or cooled by the engine cooling system.
Section 5 · 5.1.1 – Air Compressor ↗PDF, 41 MB -
What does the air compressor governor control?
When the air compressor pumps air into the storage tanks
The governor decides when the compressor pumps and when it stops.
Section 5 · 5.1.2 – Air Compressor Governor ↗PDF, 41 MB -
At about what tank pressure does the governor stop the compressor from pumping (cut-out)?
Around 125 psi
The cut-out level is around 125 psi.
Section 5 · 5.1.2 – Air Compressor Governor ↗PDF, 41 MB -
At about what tank pressure does the governor let the compressor start pumping again (cut-in)?
Around 100 psi
When tank pressure falls to the cut-in pressure, about 100 psi, the compressor starts again.
Section 5 · 5.1.2 – Air Compressor Governor ↗PDF, 41 MB -
How much air do the storage tanks hold?
Enough to use the brakes several times, even if the compressor stops
The tanks keep enough air for several brake applications if the compressor fails.
Section 5 · 5.1.3 – Air Storage Tanks ↗PDF, 41 MB -
Why is water in compressed air bad for the air brake system?
It can freeze in cold weather and cause brake failure
Water in the system can freeze and stop the brakes from working.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
Where do water and oil tend to collect in the air brake system?
In the bottom of the air tank
Water and compressor oil settle in the bottom of the tank, where the drain valve is.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
How is a manual air tank drain operated?
By turning a quarter turn or by pulling a cable
Manual drain valves open with a quarter turn or a pull on a cable.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
With manual drain valves, when must you drain the air tanks yourself?
At the end of each day of driving
Manually drained tanks must be emptied by the driver at the end of each driving day.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
How are water and oil removed from automatic air tank drains?
They are expelled automatically
Automatic drains expel water and oil by themselves, and some may also be drained by hand.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
Why do some automatic air tank drains have electric heating devices?
To help prevent freezing of the drain in cold weather
The heaters keep the automatic drain from freezing in cold weather.
Section 5 · 5.1.4 – Air Tank Drains ↗PDF, 41 MB -
What does an alcohol evaporator do?
Puts alcohol into the air system to reduce the risk of ice
Alcohol in the air lowers the risk of ice forming in valves and other parts in cold weather.
Section 5 · 5.1.5 – Alcohol Evaporator ↗PDF, 41 MB -
During cold weather, how often should you check and fill the alcohol container?
Every day
Check the alcohol container and fill it as needed every day in cold weather.
Section 5 · 5.1.5 – Alcohol Evaporator ↗PDF, 41 MB -
If your system has an alcohol evaporator but manual drains, do you still drain the air tanks daily?
Yes, daily draining is still needed to remove water and oil
The evaporator does not remove water and oil, so daily draining is still needed.
Section 5 · 5.1.5 – Alcohol Evaporator ↗PDF, 41 MB -
Where is the safety relief valve installed?
In the first tank the air compressor pumps air to
The safety valve sits in the first tank that receives air from the compressor.
Section 5 · 5.1.6 – Safety Valve ↗PDF, 41 MB -
At what pressure is the safety relief valve usually set to open?
150 psi
The safety valve protects the system from too much pressure and usually opens at 150 psi.
Section 5 · 5.1.6 – Safety Valve ↗PDF, 41 MB -
The safety valve releases air. What does that mean?
Something is wrong and a mechanic should fix it
A safety valve that lets air out signals a fault to be fixed by a mechanic.
Section 5 · 5.1.6 – Safety Valve ↗PDF, 41 MB -
What is another name for the brake pedal in an air brake system?
The foot valve or treadle valve
The brake pedal is also called the foot valve or treadle valve.
Section 5 · 5.1.7 – The Brake Pedal ↗PDF, 41 MB -
What happens when you push the brake pedal down harder?
More air pressure is applied
Pushing the pedal harder applies more air pressure to the brakes.
Section 5 · 5.1.7 – The Brake Pedal ↗PDF, 41 MB -
What can happen if you press and release the brake pedal unnecessarily?
Air can go out faster than the compressor can replace it
Each release lets air out, and too much fanning can drop pressure so low the brakes will not work.
Section 5 · 5.1.7 – The Brake Pedal ↗PDF, 41 MB -
What is the most common type of foundation brake?
The s-cam drum brake
Foundation brakes are at each wheel, and the s-cam drum brake is the most common.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
Where are brake drums located?
On each end of the vehicle’s axles
Drums are at each end of the axles, and the wheels are bolted to them.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
What does the friction of the brake shoes against the drum create besides slowing the vehicle?
Heat
Friction slows the vehicle and creates heat, and too much heat can make the brakes stop working.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
On s-cam brakes, what does the push rod move when air enters the brake chamber?
The slack adjuster, which twists the brake camshaft
Air pushes the rod out, moving the slack adjuster and twisting the camshaft to turn the s-cam.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
When you release the brake pedal on s-cam brakes, what pulls the brake shoes away from the drum?
A spring
The s-cam rotates back and a spring pulls the shoes off the drum so the wheels roll freely.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
In a wedge brake, what does the brake chamber push rod push?
A wedge directly between the ends of two brake shoes
The wedge shoves the shoes apart and against the inside of the drum.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
In air-operated disc brakes, what is used instead of the s-cam?
A power screw
The slack adjuster turns a power screw that clamps the rotor like a large c-clamp.
Section 5 · 5.1.8 – Foundation Brakes ↗PDF, 41 MB -
In a dual air brake system, how are the supply pressure gauges set up?
A gauge for each half of the system, or one gauge with two needles
Dual systems have a gauge for each half, or a single gauge with two needles.
Section 5 · 5.1.9 – Supply Pressure Gauges ↗PDF, 41 MB -
What does the application pressure gauge show?
How much air pressure you are applying to the brakes
The application gauge shows the pressure going to the brakes, and not all vehicles have one.
Section 5 · 5.1.10 – Application Pressure Gauge ↗PDF, 41 MB -
You need more and more application pressure to hold the same speed. What does that mean?
The brakes are fading
Rising application pressure at a steady speed means brake fade, so slow down and use a lower gear.
Section 5 · 5.1.10 – Application Pressure Gauge ↗PDF, 41 MB -
Is a low air pressure warning signal required on vehicles with air brakes?
Yes, it is required
Every vehicle with air brakes must have a low air pressure warning signal.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
Before tank pressure falls below what level must a visible low air pressure warning come on?
55 psi
A warning you can see must come on before the tanks fall below 55 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
On older vehicles, at what point must the low air pressure warning come on?
One half the compressor governor cutout pressure
On older vehicles the warning point may be half the governor cutout pressure.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
What does the low air pressure warning usually look like?
A red light, sometimes with a buzzer
The warning is usually a red light, and a buzzer may also sound.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
What does a “wig wag” do?
Drops a mechanical arm into your view when pressure falls below 55 psi
The wig wag is a warning device that drops an arm into view at low pressure.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
When will a manual reset wig wag stay in the “out of view” position?
Only when the system pressure is above 55 psi
You must place it out of view by hand, and it stays only once pressure is above 55 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
On large buses, at what pressure is it common for low pressure warning devices to signal?
80-85 psi
Large buses commonly warn earlier, at 80-85 psi.
Section 5 · 5.1.11 – Low Air Pressure Warning ↗PDF, 41 MB -
How does the air brake system turn on the brake lights?
With an electric switch that works by air pressure
The stop light switch is an electric switch worked by air pressure.
Section 5 · 5.1.12 – Stop Light Switch ↗PDF, 41 MB -
On an older vehicle with a front brake limiting valve, what does the “slippery” position do?
Cuts the normal air pressure to the front brakes by half
In “slippery” the limiting valve halves the air to the front brakes.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗PDF, 41 MB -
Vehicles made before what year may have a front brake limiting valve with a control in the cab?
1975
Some older vehicles, made before 1975, have this valve and cab control.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗PDF, 41 MB -
What position should the front brake limiting control be in?
Normal, to have normal stopping power
Limiting valves reduce stopping power, so keep the control in normal.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗PDF, 41 MB -
Are front wheel skids from braking likely on ice?
No, tests show they are not likely even on ice
Front wheel braking is good under all conditions, and front skids are not likely even on ice.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗PDF, 41 MB -
When do automatic front wheel limiting valves let full air go to the front brakes?
When the brakes are put on very hard, at 55 psi or more application pressure
Automatic limiting valves cut front brake air except at 55 psi or more application pressure, and the driver cannot control them.
Section 5 · 5.1.13 – Front Brake Limiting Valve ↗PDF, 41 MB -
Why must emergency and parking brakes be held on by mechanical force?
Because air pressure can eventually leak away
Air can leak away over time, so these brakes are held on mechanically, usually by springs.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
How do spring brakes work while you drive?
Powerful springs are held back by air pressure
Air holds the springs back, and when air is removed the springs put on the brakes.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
A leak causes all the air in the brake system to be lost. What happens?
The springs put on the brakes
Losing all the air lets the springs apply the brakes.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
In what pressure range do tractor and straight truck spring brakes come fully on?
20 to 45 psi (typically 20 to 30 psi)
Spring brakes come fully on when air pressure drops to 20 to 45 psi, typically 20 to 30.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
When the low air pressure warning light and buzzer first come on, what should you do?
Bring the vehicle to a safe stop right away
Do not wait for the spring brakes. Stop safely while you can still control the brakes.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
What does the braking power of spring brakes depend on?
The brakes being in adjustment
If brakes are out of adjustment, neither the regular nor the emergency/parking brakes work right.
Section 5 · 5.1.14 – Spring Brakes ↗PDF, 41 MB -
In newer vehicles, what does the parking brake control look like?
A diamond-shaped, yellow, push-pull knob
Newer air brake vehicles use a yellow, diamond-shaped push-pull knob.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
How do you put on the parking brakes with the push-pull knob?
Pull the knob out
Pull out to apply the spring brakes, push in to release them.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
Why should you never push the brake pedal down when the spring brakes are on?
The combined forces of the springs and air pressure could damage the brakes
Spring force plus air pressure together can damage the brakes, so do not press the pedal with spring brakes on.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
What is a modulating control valve?
A dash handle that applies the spring brakes gradually
It is spring-loaded so you can feel the braking, and moving it farther applies the spring brakes harder.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
How do you park a vehicle with a modulating control valve?
Move the lever as far as it will go and lock it in place
Move the lever fully and hold it with the locking device.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
On a vehicle with dual parking control valves, what does the separate air tank allow you to do?
Release the spring brakes to move in an emergency
Some vehicles, such as buses, can use a separate tank to release the spring brakes after main air is lost.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
Why must you plan carefully when moving with air from the separate tank of dual parking control valves?
There is only enough air to release the spring brakes a few times
The separate tank has only enough air for a few releases, so you could stop in a dangerous spot.
Section 5 · 5.1.15 – Parking Brake Controls ↗PDF, 41 MB -
Truck tractors with air brakes built on or after what date must have antilock brakes?
March 1, 1997
ABS is required on air-braked truck tractors built on or after March 1, 1997.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
Other air-braked vehicles, such as trucks, buses, and trailers, built on or after what date must have ABS?
March 1, 1998
Trucks, buses, trailers, and converter dollies with air brakes need ABS if built on or after March 1, 1998.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
How can you find out whether your vehicle is equipped with ABS?
Check the certification label for the date of manufacture
The manufacture date on the certification label tells you if ABS was required.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
What color are ABS malfunction lamps?
Yellow
Vehicles with ABS have yellow malfunction lamps.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
Where is the ABS malfunction lamp on a trailer?
On the left side, on the front or rear corner
Trailers have their yellow ABS lamps on the left side, front or rear corner.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
On older ABS systems, how long could the malfunction lamp stay on after start-up?
Until you are driving over five mph
Newer lamps go out quickly after the bulb check, but older ones could stay on until you pass five mph.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
The ABS lamp stays on after the bulb check. What does that mean?
You may have lost ABS control at one or more wheels
A lamp that stays on, or comes on while moving, signals a possible loss of ABS at some wheels.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
On an older towed unit, what can you look for to tell if it has ABS?
The electronic control unit and wheel speed sensor wires
Look under the vehicle for the ECU and sensor wires coming from the back of the brakes.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
How does ABS change your normal braking capability?
It does not decrease or increase it
ABS is an addition to normal brakes and only activates when wheels are about to lock.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
Does ABS shorten your stopping distance?
Not necessarily, but it helps keep control in hard braking
ABS helps you keep control during hard braking but does not necessarily stop you sooner.
Section 5 · 5.1.16 – Antilock Braking Systems (ABS) ↗PDF, 41 MB -
What is a dual air brake system?
Two separate air brake systems that use a single set of brake controls
Each half has its own tanks, hoses, and lines, but the driver uses one set of controls.
Section 5 · 5.2 – Dual Air Brake ↗PDF, 41 MB -
In a dual air brake system, what does the primary system typically operate?
The regular brakes on the rear axle or axles
One system typically works the rear axle brakes, the other the front axle and possibly one rear axle.
Section 5 · 5.2 – Dual Air Brake ↗PDF, 41 MB -
Before driving a vehicle with a dual air system, how much pressure should build up?
At least 100 psi in both the primary and secondary systems
Let the compressor build a minimum of 100 psi in both systems before you drive.
Section 5 · 5.2 – Dual Air Brake ↗PDF, 41 MB -
In a dual system, the warning light and buzzer shut off when pressure in both systems reaches a manufacturer value. That value must be greater than what?
55 psi
The shut-off value set by the manufacturer must be greater than 55 psi.
Section 5 · 5.2 – Dual Air Brake ↗PDF, 41 MB -
One air system of a dual system is very low on pressure. What happens?
Either the front or rear brakes will not fully work, so stopping takes longer
With one half low, part of the brakes are weak. Stop safely and have the system fixed.
Section 5 · 5.2 – Dual Air Brake ↗PDF, 41 MB
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