Commercial driver’s license (CDL) knowledge tests · California DMV
Air Brake Parts: Compressor, Tanks and Foundation Brakes
Questions and answers
-
What do air brakes use to make the brakes work?
Compressed air
Air brakes work with compressed air.
Section 5 · SECTION 5: AIR BRAKES ↗ -
Air brakes are really three braking systems. Which three?
Service brake, parking brake, and emergency brake
An air brake system is made up of the service, parking, and emergency brake systems.
Section 5 · SECTION 5: AIR BRAKES ↗ -
Which system applies and releases the brakes when you use the brake pedal during normal driving?
The service brake system
The service brake system is the one worked by the brake pedal in normal driving.
Section 5 · SECTION 5: AIR BRAKES ↗ -
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 brake systems.
Section 5 · SECTION 5: AIR BRAKES ↗ -
For CDL purposes, which two components must a vehicle’s air brake system contain?
Air gauges and low pressure warning device(s)
Air gauges and low pressure warning devices are required and are checked in the vehicle inspection test.
Section 5 · SECTION 5: AIR BRAKES ↗ -
The vehicle you bring to your road test has no air gauges or low pressure warning device. What is the result?
A “No Air Brakes” (“L”) restriction on your CDL
Without those components the vehicle is not considered to have air brakes, and the license gets the “L” restriction.
Section 5 · SECTION 5: AIR BRAKES ↗ -
A full service brake application must deliver at least what share of the remaining air reservoir pressure to all brake chambers?
90 percent
With the brakes applied, not less than 90 percent of the remaining reservoir pressure must reach every brake chamber.
Section 5 · SECTION 5: AIR BRAKES ↗ -
What does the air compressor do?
Pumps air into the air storage tanks
The compressor pumps air into the air storage tanks, also called reservoirs.
Section 5 · 5.1.1 – Air Compressor ↗ -
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 ↗ -
How may an air compressor be cooled?
By air or by the engine cooling system
A compressor may be air cooled or cooled by the engine cooling system.
Section 5 · 5.1.1 – Air Compressor ↗ -
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 driving.
Section 5 · 5.1.1 – Air Compressor ↗ -
What does the air compressor governor control?
When the air compressor pumps air into the storage tanks
The governor decides when the compressor pumps.
Section 5 · 5.1.2 – Air Compressor Governor ↗ -
At about what pressure does the governor stop the compressor from pumping (the “cut-out” level)?
Around 125 psi
The cut-out level is around 125 psi.
Section 5 · 5.1.2 – Air Compressor Governor ↗ -
At about what pressure does the governor let the compressor start pumping again (the “cut-in” pressure)?
Around 100 psi
The cut-in pressure is around 100 psi.
Section 5 · 5.1.2 – Air Compressor Governor ↗ -
Tank pressure has risen to the cut-out level. What does the governor do?
Stops the compressor from pumping air
At cut-out the governor stops the compressor from pumping.
Section 5 · 5.1.2 – Air Compressor Governor ↗ -
The air compressor stops working. What do the air storage tanks still allow?
The brakes to be used several times
The tanks hold enough air for several brake applications even without the compressor.
Section 5 · 5.1.3 – Air Storage Tanks ↗ -
What does compressed air usually have in it that is bad for the air brake system?
Some water and some compressor oil
Compressed air usually carries some water and compressor oil.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
Why is water in the air tanks dangerous in cold weather?
It can freeze and cause brake failure
Water in the system can freeze and make the brakes fail.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
Where is the drain valve on an air tank?
In the bottom of the tank
Water and oil collect in the bottom of the tank, and each tank has a drain valve there.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
How is a manual air tank drain valve operated?
By turning it a quarter turn or pulling a cable
A manual drain is worked with a quarter turn or a pull cable.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
Your air tanks have manual drain valves. How often must you drain them?
At the end of each day of driving
With manual drains you must drain the tanks yourself at the end of each day of driving.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
What does an automatic air tank drain do?
Expels the water and oil automatically
An automatic drain expels water and oil by itself. Such tanks may also be equipped for manual draining.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
What are the electric heating devices on some automatic air tank drains for?
To help prevent the automatic drain from freezing in cold weather
The heaters keep the automatic drain from freezing.
Section 5 · 5.1.4 – Air Tank Drains ↗ -
What does an alcohol evaporator do?
Puts alcohol into the air system to reduce the risk of ice
Alcohol in the air system lowers the risk of ice in the valves and other parts in cold weather.
Section 5 · 5.1.5 – Alcohol Evaporator ↗ -
How often should you check and fill the alcohol container during cold weather?
Every day
Check the alcohol container and fill it as needed, every day in cold weather.
Section 5 · 5.1.5 – Alcohol Evaporator ↗ -
Your vehicle has an alcohol evaporator and manual drain valves. Do you still need to drain the air tanks daily?
Yes, daily draining is still needed to get rid of water and oil
An evaporator does not replace daily draining, unless the system has automatic drain valves.
Section 5 · 5.1.5 – Alcohol Evaporator ↗ -
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 the compressor fills.
Section 5 · 5.1.6 – Safety Valve ↗ -
At what pressure is the safety valve usually set to open?
150 psi
The safety valve is usually set to open at 150 psi.
Section 5 · 5.1.6 – Safety Valve ↗ -
The safety valve releases air. What does that mean?
Something is wrong and a mechanic should fix the fault
The valve protects the system from too much pressure, so if it opens there is a fault to repair.
Section 5 · 5.1.6 – Safety Valve ↗ -
What does the safety valve protect the tank and the rest of the system from?
Too much pressure
The safety valve guards against excessive pressure.
Section 5 · 5.1.6 – Safety Valve ↗ -
What are other names for the brake pedal in an air brake system?
Foot valve or treadle valve
The brake pedal is also called the foot valve or treadle valve.
Section 5 · 5.1.7 – Brake Pedal ↗ -
What happens when you push the brake pedal down harder?
More air pressure is applied
Pushing harder applies more air pressure to the brakes.
Section 5 · 5.1.7 – Brake Pedal ↗ -
What happens to the air in the system each time you release the brakes?
Some compressed air goes out and tank pressure is reduced
Releasing the brakes lets some air out, and the compressor has to make it up.
Section 5 · 5.1.7 – Brake Pedal ↗ -
Why should you not press and release the brake pedal unnecessarily?
It can let air out faster than the compressor can replace it
Needless pumping can use air faster than it is replaced, and with too little pressure the brakes will not work.
Section 5 · 5.1.7 – Brake Pedal ↗ -
What is the most common type of foundation brake?
The S-cam drum brake
S-cam drum brakes are the most common. Wedge and disc brakes are less common.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
Where are the brake drums located?
On each end of the vehicle’s axles
The drums are on each end of the axles, and the wheels are bolted to them.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
In a drum brake, what is pushed against the inside of the drum to stop the vehicle?
The brake shoes and linings
The shoes and linings press against the inside of the drum, and the friction slows the vehicle.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
What can too much heat do to drum brakes?
Make the brakes stop working
A drum can take only so much heat, and too much can make the brakes stop working.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
In an S-cam brake, what does air pressure in the brake chamber do when you push the pedal?
Pushes the rod out, moving the slack adjuster and twisting the brake camshaft
The rod moves the slack adjuster, which twists the camshaft and turns the S-cam.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
What does the S-cam do to the brake shoes?
Forces them away from one another and against the inside of the drum
The turning S-cam spreads the shoes and presses them against the drum.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
In an S-cam brake, what pulls the shoes away from the drum when you release the pedal?
A spring
When the S-cam rotates back, a spring pulls the shoes away so the wheels roll freely.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
How does the CamLaster brake differ from a traditional S-cam brake in the way it is adjusted?
It has a completely internal adjustment system
The CamLaster keeps itself in adjustment internally, while S-cam brakes need an external slack adjuster.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
How does a CamLaster brake apply the shoes to the drum?
It slides the shoes down an inclined ramp on a cam
Its cam design slides the shoes down an inclined ramp so they contact the drum evenly.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
How does a wedge brake push the shoes against the drum?
The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
The wedge shoves the two shoes apart and against the inside of the drum.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
How are wedge brakes adjusted?
They may be self-adjusting or may require manual adjustment
Some wedge brakes adjust themselves and others need manual adjustment.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
What is used in an air-operated disc brake instead of the S-cam?
A “power screw”
The power screw clamps the disc or rotor between the lining pads of a caliper.
Section 5 · 5.1.8 – Foundation Brakes ↗ -
What does the supply pressure gauge tell you?
How much pressure is in the air tanks
The supply pressure gauge is connected to the air tank and shows tank pressure.
Section 5 · 5.1.9 – Supply Pressure Gauges ↗ -
How are supply pressure gauges arranged on a vehicle with a dual air brake system?
A gauge for each half of the system, or a single gauge with 2 needles
Each half of a dual system has its own gauge or its own needle.
Section 5 · 5.1.9 – Supply Pressure Gauges ↗ -
What does the application pressure gauge show?
How much air pressure you are applying to the brakes
This gauge, which not all vehicles have, shows the pressure being applied to the brakes.
Section 5 · 5.1.10 – Application Pressure Gauge ↗ -
You need more and more application pressure to hold the same speed. What does that mean?
The brakes are fading
Increasing application pressure to hold the same speed is a sign of brake fade.
Section 5 · 5.1.10 – Application Pressure Gauge ↗ -
Your brakes are fading on a downgrade. What should you do?
Slow down and use a lower gear
When the brakes fade, slow down and use a lower gear.
Section 5 · 5.1.10 – Application Pressure Gauge ↗ -
Besides brake fade, what can cause the need for increased application pressure?
Brakes out of adjustment, air leaks, or mechanical problems
Adjustment problems, air leaks, and mechanical faults can also raise the pressure you need.
Section 5 · 5.1.10 – Application Pressure Gauge ↗
Questions and answers by topic
- Licenses, endorsements and tests105
- Disqualifications and state rules110
- Vehicle inspection80
- Basic control and shifting gears47
- Seeing and communicating46
- Controlling your speed62
- Managing space40
- Seeing hazards49
- Distracted and Aggressive Driving71
- Night, Fog, Winter and Hot Weather124
- Railroad Crossings and Mountain Driving65
- Emergencies, ABS and Skids102
- Accident Procedures and Fires49
- Alcohol, Drugs and Illness41
- Hazardous Materials Rules for All Drivers42
- Transporting Cargo Safely79
- Bus Inspection, Loading and Hazardous Materials74
- Driving a Bus: Stops, Crossings and Prohibited Practices56
- Warnings, Spring Brakes, ABS and Dual Systems59
- Inspecting Air Brake Systems61
- Using Air Brakes51
- Driving Combination Vehicles Safely50
- Combination Vehicle Air Brakes and ABS59
- Coupling and Uncoupling63
- Inspecting a Combination Vehicle24
- Pulling Double and Triple Trailers16
- Coupling, Inspecting and Brake Checks for Doubles34
- Tank Vehicles62
- HazMat basics and who does what48
- Hazard classes, labels and lists64
- Shipping papers and package markings49
- Placards and placard tables40
- Loading and unloading HazMat58
- Bulk packaging and tanks30
- HazMat driving and parking rules72
- HazMat emergencies and reporting60
- Danger zones and mirrors34
- Loading and unloading students47
- Emergency exit and evacuation34
- Railroad-highway crossings40
- Student management, ABS and special situations47