What is the air compressor’s job in an air brake system?
Correct answer: Pump air into the storage tanks
The compressor supplies compressed air to the reservoirs used by the brake system.
Source: CA Commercial Driver Handbook § 5.1.1 Air Compressor ↗
At approximately what tank pressure does the governor normally cut out the air compressor?
Correct answer: 125 psi
The governor normally stops compressor loading near 125 psi, although manufacturer specifications may vary.
Source: CA Commercial Driver Handbook § 5.1.2 Air Compressor Governor ↗
At approximately what pressure does the governor normally allow the compressor to cut in again?
Correct answer: 100 psi
As reservoir pressure falls to roughly 100 psi, the governor normally restarts compressor loading.
Source: CA Commercial Driver Handbook § 5.1.2 Air Compressor Governor ↗
Why must moisture and oil be removed from air storage tanks?
Correct answer: Water can freeze and interfere with braking
Water and oil collect in the tanks; frozen water can obstruct the system and cause brake failure.
Source: CA Commercial Driver Handbook § 5.1.4 Air Tank Drains ↗
When should manually drained air tanks normally be emptied?
Correct answer: At the end of each driving day
Manual tank drains should be opened completely at the end of each day of driving.
Source: CA Commercial Driver Handbook § 5.1.4 Air Tank Drains ↗
What does the air-system safety valve protect against?
Correct answer: Excessive pressure
The valve protects the first tank and the rest of the system from overpressure and usually opens near 150 psi.
Source: CA Commercial Driver Handbook § 5.1.6 Safety Valve ↗
Why should a driver avoid unnecessary repeated brake-pedal applications?
Correct answer: They can use air faster than the compressor replaces it
Each release exhausts compressed air, so repeated applications may lower pressure to an unsafe level.
Source: CA Commercial Driver Handbook § 5.1.7 Brake Pedal ↗
What can increasing application pressure needed to hold the same speed indicate?
Correct answer: The brakes may be fading
A rising application-pressure requirement can signal brake fade, poor adjustment, leaks, or mechanical trouble.
Source: CA Commercial Driver Handbook § 5.1.10 Application Pressure Gauge ↗
Under California's CDL handbook low-air test, the visible warning must activate before pressure falls below which threshold?
Correct answer: 55 psi
The handbook test requires the warning before pressure falls below 55 psi. Many systems warn earlier; follow the vehicle specifications and stop safely when a low-air warning occurs.
Source: CA Commercial Driver Handbook § 5.3.3.2 Low Air Warning ↗
What holds spring brakes off while a vehicle is being driven normally?
Correct answer: Air pressure
Air pressure compresses the powerful springs; loss of pressure lets the springs apply the brakes.
Source: CA Commercial Driver Handbook § 5.1.14 Spring Brakes ↗
At what approximate pressure range do tractor and straight-truck spring brakes come fully on?
Correct answer: 20 to 45 psi
Spring brakes normally apply automatically as pressure falls into the 20-45 psi range.
Source: CA Commercial Driver Handbook § 5.1.14 Spring Brakes ↗
On a newer air-brake vehicle, how is the parking brake normally applied?
Correct answer: Pull out the yellow diamond-shaped control
Pulling out the yellow push-pull knob releases air from the spring brakes so they apply.
Source: CA Commercial Driver Handbook § 5.1.15 Parking Brake Controls ↗
Why should the foot brake not be pressed while spring brakes are already applied?
Correct answer: Combined spring and air forces may damage the brakes
Some systems can be damaged when air pressure adds to the force of applied spring brakes.
Source: CA Commercial Driver Handbook § 5.1.15 Parking Brake Controls ↗
If the ABS malfunction lamp stays on, what braking capability remains?
Correct answer: Normal braking remains, but ABS needs service
An ABS fault does not remove normal brakes; drive normally and arrange service promptly.
Source: CA Commercial Driver Handbook § 5.4.2 Braking With ABS ↗
With the vehicle on level ground, wheels chocked and parking brakes released, you pull a reachable S-cam slack adjuster firmly by hand. What movement at the push-rod connection suggests it needs attention?
Correct answer: More than about one inch at the push-rod connection
With wheels chocked and parking brakes released, manually pulling a reachable S-cam slack adjuster more than about one inch at the push-rod connection signals trouble. This is not a universal applied push-rod stroke limit. Have faults repaired; routinely adjusting automatic adjusters is not a substitute for repair.
Source: CA Commercial Driver Handbook § 5.3.2 Check Slack Adjusters on S-cam Brakes ↗
During a one-minute applied leakage test, what is the maximum pressure drop for a single vehicle?
Correct answer: 3 psi
Secure the vehicle, charge the system, shut off the engine, and release the parking brakes. Fully apply the foot brake and let the initial pressure drop stabilize before timing one minute. A single vehicle must not then lose more than 3 psi.
Source: CA Commercial Driver Handbook § 5.3.3.1 Applied Air Leakage Test ↗
During a one-minute applied leakage test, what is the maximum drop for a two-vehicle air-brake combination?
Correct answer: 4 psi
With both vehicles air-braked, secure the combination, charge the system, shut off the engine, and release parking brakes. Fully apply the foot brake. After the initial drop stabilizes, loss must not exceed 4 psi in one minute.
Source: CA Commercial Driver Handbook § 5.3.3.1 Applied Air Leakage Test ↗
With all brakes released, what is the maximum one-minute static pressure loss for a single vehicle?
Correct answer: 2 psi
Secure the vehicle against rolling, charge the system, shut off the engine, and release all brakes. Let pressure stabilize, then time one minute. A single vehicle must not lose more than 2 psi.
Source: CA Commercial Driver Handbook § 5.3.3 Static Air Leakage Test ↗
For a dual air system at normal idle, what buildup demonstrates the expected rate?
Correct answer: About 85 to 100 psi within 45 seconds
A dual system should normally build from about 85 to 100 psi within 45 seconds.
Source: CA Commercial Driver Handbook § 5.3.3 Check Rate of Air Pressure Buildup ↗
How should service brakes be checked after normal air pressure is restored?
Correct answer: Move forward slowly at about 5 mph and apply them firmly
A low-speed firm application can reveal pulling, unusual feel, or delayed stopping.
Source: CA Commercial Driver Handbook § 5.3.3 Test Service Brakes ↗
On a vehicle without ABS, what defines controlled braking during an emergency stop?
Correct answer: Brake as hard as possible without locking the wheels
Use firm braking just short of wheel lock and keep steering movements small. If the wheels lock, release enough to regain control and reapply. With ABS, brake firmly as needed without pumping the pedal.
Source: CA Commercial Driver Handbook § 5.4.3 Emergency Stops ↗
Which extra factor is part of total stopping distance for an air-brake vehicle?
Correct answer: Brake lag distance
Air needs time to reach the brakes, adding brake lag to perception, reaction, and braking distances.
Source: CA Commercial Driver Handbook § 5.4.4 Stopping Distance ↗
At 55 mph on dry pavement, approximately how much distance can air-brake lag add?
Correct answer: 32 feet
The handbook estimates about 32 feet of brake-lag distance at 55 mph.
Source: CA Commercial Driver Handbook § 5.4.4 Stopping Distance ↗
What is a primary cause of brake fade on a long downgrade?
Correct answer: Excessive service-brake use and heat
Overuse creates heat that reduces lining friction and expands drums, weakening braking.
Source: CA Commercial Driver Handbook § 5.4.5 Brake Fading or Failure ↗
What should a driver do when the low-air warning activates on the road?
Correct answer: Stop and park safely as soon as possible while controlled braking remains
A warning may indicate a leak; stop safely before pressure falls far enough for spring brakes to apply.
Source: CA Commercial Driver Handbook § 5.4.7 Low Air Pressure ↗
Before moving a vehicle with a dual air brake system, what should you confirm on both pressure gauges?
Correct answer: Both systems have reached at least 100 psi
The handbook instructs drivers to allow pressure to build to at least 100 psi in both systems before driving. Check both gauges rather than assuming one charged circuit means the other is ready.
Source: CA Commercial Driver Handbook § 5.2 Dual Air Brake Systems ↗
One circuit of a dual air brake system loses pressure while you are driving. Why must you stop safely and obtain repairs?
Correct answer: The remaining circuit does not provide normal braking at all wheels
A failure in one circuit can leave part of the vehicle without working service brakes. Stopping distance increases, so bring the vehicle to a safe stop and have the system repaired before continuing.
Source: CA Commercial Driver Handbook § 5.2 Dual Air Brake Systems ↗
What is the purpose of the one-way check valve between the compressor and the first air reservoir?
Correct answer: Keep stored air from escaping back toward a compressor leak
The check valve allows air into the reservoir but prevents reverse flow. If the compressor develops a leak, stored reservoir air should not escape back through that connection.
Source: CA Commercial Driver Handbook § 5.3.1 Check Air Compressor Drive Belt / One-Way Check Valve ↗
An automatic slack adjuster repeatedly has excessive stroke. What is the appropriate response?
Correct answer: Have the underlying brake problem diagnosed and repaired
Excessive stroke with an automatic adjuster can indicate a mechanical problem. Routine manual adjustment can conceal the defect and is not a substitute for qualified repair following the manufacturer’s procedures.
Source: CA Commercial Driver Handbook § 5.3.2 Check Slack Adjusters on S-cam Brakes ↗
After its normal startup check, an ABS warning lamp comes on while the vehicle is moving. What does this suggest?
Correct answer: A possible ABS malfunction that needs attention
An ABS malfunction lamp that remains on after the appropriate check or comes on during travel can indicate an ABS fault. Normal braking remains, but the fault needs service; follow the vehicle-specific lamp-check procedure.
Source: CA Commercial Driver Handbook § 5.1.16 Antilock Braking Systems ↗
Which benefit should you expect from ABS during hard braking?
Correct answer: Reduced wheel locking, helping you retain vehicle control
ABS helps prevent wheel lock during hard braking. It supports control but does not necessarily shorten stopping distance and does not replace careful speed or following-distance choices.
Source: CA Commercial Driver Handbook § 5.1.16 Antilock Braking Systems ↗
For an air-braked truck towing a trailer with electric brakes, which one-minute applied-air leakage limit does the handbook specify?
Correct answer: 3 psi
The 4 psi two-vehicle limit applies when both vehicles have air brakes. When the towed vehicle has electric or surge brakes, the handbook specifies 3 psi. Time the applied test after the initial pressure drop stabilizes with the vehicle secured.
Source: CA Commercial Driver Handbook § 5.3.3.1 Applied Air Leakage Test ↗
During a brake inspection, you find oil or grease on a brake lining. How should this be treated?
Correct answer: As a brake defect that needs correction
Brake linings should not be loose or contaminated with oil or grease. Normal air pressure does not make a defective friction surface safe; the defect needs correction.
Source: CA Commercial Driver Handbook § 5.3.2 Check Brake Drums, Linings, and Hoses ↗
An older truck has a manual front brake limiting valve marked NORMAL and SLIPPERY. Which setting normally preserves full front braking?
Correct answer: NORMAL
The manual limiting valve reduces front brake pressure in the SLIPPERY position and can reduce stopping ability. The handbook says to leave this type of control in NORMAL for normal stopping power.
Source: CA Commercial Driver Handbook § 5.1.13 Front Brake Limiting Valve ↗
What should you look for when inspecting the flexible air hoses connected to brake chambers?
Correct answer: Damage or wear caused by rubbing
Inspect brake hoses for damage and wear from rubbing. A hose problem can lead to air loss; do not dismiss it because pressure is currently normal or disconnect a pressurized line as a routine inspection step.
Source: CA Commercial Driver Handbook § 5.3.2 Check Brake Drums, Linings, and Hoses ↗
Which three functions make up the air-brake system described in the handbook?
Correct answer: Service, parking, and emergency braking
The functions serve normal stops, parking, and brake-system failures.
Source: CA Commercial Driver Handbook § 5 Air Brakes — Three Systems ↗
In normal service braking, pressing the pedal farther does what?
Correct answer: Sends greater application pressure to the brakes
Pedal force controls service-brake application pressure.
Source: CA Commercial Driver Handbook § 5.1.7 Brake Pedal ↗
An air compressor has its own oil supply rather than engine lubrication. What should you check before driving?
Correct answer: Its oil level
A separately lubricated compressor needs its own oil check.
Source: CA Commercial Driver Handbook § 5.1.1 Air Compressor ↗
Does an alcohol evaporator eliminate the need to drain manually drained air tanks?
Correct answer: No, tank drainage is still required
The evaporator helps reduce freezing risk; it does not remove accumulated water and oil.
Source: CA Commercial Driver Handbook § 5.1.5 Alcohol Evaporator ↗
The air-system safety valve releases air. What does this indicate?
Correct answer: A fault requiring mechanical attention
The safety valve is a pressure safeguard, not the ordinary compressor control.
Source: CA Commercial Driver Handbook § 5.1.6 Safety Valve ↗
What does the S-cam force against the inside of the drum?
Correct answer: The brake shoes and linings
Contact between lining and drum creates the friction used to slow the wheel.
Source: CA Commercial Driver Handbook § 5.1.8 S-cam Brakes ↗
In an air-operated disc brake, what is clamped between the lining pads?
Correct answer: The disc or rotor
Disc brakes produce braking friction at the rotor rather than inside a drum.
Source: CA Commercial Driver Handbook § 5.1.8 Disc Brakes ↗
What does the supply-pressure gauge show?
Correct answer: Pressure available in the air tanks
Do not confuse stored air pressure with an application-pressure reading.
Source: CA Commercial Driver Handbook § 5.1.9 Supply Pressure Gauges ↗
How are following drivers warned when air service brakes are applied?
Correct answer: An air-pressure-operated electrical switch illuminates the brake lights
The brake-light switch communicates the braking action to traffic behind.
Source: CA Commercial Driver Handbook § 5.1.12 Stop Light Switch ↗
With a safe test area and seat belt fastened, how do you check whether the parking brake holds?
Correct answer: Set it and gently pull against it in a low gear
Use a gentle holding test rather than forceful acceleration.
Source: CA Commercial Driver Handbook § 5.3 Test Parking Brake ↗
In a settled, one-minute static leakage test with brakes released, what is the maximum loss for two air-braked vehicles?
Correct answer: 3 psi
Secure against rolling, charge the system, shut off the engine and release all brakes. Time one minute after stabilization; this is not the applied test.
Source: CA Commercial Driver Handbook § 5.3 Test Air Leakage Rate — Static Leakage ↗
For three or more air-braked vehicles, what maximum one-minute pressure drop does the handbook allow in the static test?
Correct answer: 5 psi
Secure against rolling, charge the system, shut off the engine and release all brakes. Measure one minute after stabilization, using the limit for this vehicle count.
Source: CA Commercial Driver Handbook § 5.3 Test Air Leakage Rate — Static Leakage ↗
In a static leakage test, the truck has air brakes but none of its towed vehicles do. What maximum loss applies?
Correct answer: 2 psi in one minute
Secure the rig, charge the air system, shut off the engine and release all brakes. After stabilization, use the single-air-braked-vehicle static limit for this configuration.
Source: CA Commercial Driver Handbook § 5.3 Static Leakage — Non-air-braked Towed Vehicles ↗
You have selected the proper low gear and a safe descent speed of 40 mph. Under the handbook’s snub-braking method, when do you release the service brakes?
Correct answer: After slowing to about 35 mph
The method reduces speed about 5 mph below the safe speed, then releases the brakes while engine braking continues.
Source: CA Commercial Driver Handbook § 5.4.6 Proper Braking Technique ↗
Why can some poorly adjusted brakes cause the other brakes to overheat?
Correct answer: The other brakes must do more than their share of the work
All brakes must contribute properly; imbalance increases the load on the working brakes.
Source: CA Commercial Driver Handbook § 5.4.5 Brake Fading or Failure ↗
