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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.1 Air Compressor, printed 5-1 (physical PDF page 95) ↗
What usual compressor cut-out range does Pennsylvania's CDL manual describe, subject to the vehicle manufacturer's specifications?
Correct answer: 120–140 psi
PUB 223 describes a usual cut-out range of 120–140 psi. During inspection, use the manufacturer's specified setting; the safety-relief valve is a different device.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.2 Air Compressor Governor; corroborating final check at section 5.3.3, printed 5-7 / physical 101, printed 5-1 (physical PDF page 95) ↗
At approximately what pressure does the governor normally allow the compressor to cut in again?
Correct answer: 100 psi
The governor normally restarts compressor loading near 100 psi. Check the manufacturer specifications for the particular vehicle; the final air-brake check uses those specified pressures.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.2 Air Compressor Governor; corroborating final check at section 5.3.3, printed 5-7 / physical 101, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.4 Air Tank Drains, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.4 Air Tank Drains, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.6 Safety Valve, printed 5-2 (physical PDF page 96) ↗
Why should a driver avoid unnecessary repeated brake-pedal applications?
Correct answer: Air use may exceed compressor output
Each release exhausts compressed air, so repeated applications may lower pressure to an unsafe level.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.7 The Brake Pedal, printed 5-2 (physical PDF page 96) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.10 Application Pressure Gauge, printed 5-3 (physical PDF page 97) ↗
During a secured pre-trip air-brake check, the low-pressure warning fails to operate as required. What is the safe decision?
Correct answer: Have the warning-system fault corrected before driving
A failed warning can leave the driver unaware of falling air pressure. Do not substitute the gauge or automatic spring-brake application for a working warning signal. Use the warning specifications applicable to the vehicle.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Low Pressure Warning Signal, printed 5-7 (physical PDF page 101) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.14 Spring Brakes, printed 5-3 (physical PDF page 97) ↗
At what approximate pressure range do tractor and straight-truck spring brakes come fully on?
Correct answer: 20 to 45 psi
The manual gives a 20–45 psi range for tractor and straight-truck spring brakes to come fully on, typically 20–30 psi. The vehicle's specifications matter. Stop safely at the low-air warning rather than waiting for the spring brakes.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.14 Spring Brakes, printed 5-3 (physical PDF page 97) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.15 Parking Brake Controls, printed 5-4 (physical PDF page 98) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.15 Parking Brake Controls — Caution, printed 5-4 (physical PDF page 98) ↗
If an ABS-only malfunction occurs and there is no separate service-brake fault, what braking capability remains?
Correct answer: Normal braking remains, but ABS needs service
An ABS malfunction does not remove normal service braking. Continue to use the brakes normally and arrange ABS service promptly; do not assume ABS protection remains available.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.2 Braking with Antilock Brakes, printed 5-8 (physical PDF page 102) ↗
On a reachable S-cam slack adjuster, what manually pulled movement at the push-rod connection suggests a need for adjustment?
Correct answer: More than about one inch
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.2 During Step 5 Walkaround Inspection — Check Slack Adjusters on S-cam Brakes, printed 5-6 (physical PDF page 100) ↗
For a single vehicle, Pennsylvania's full-foot-brake leakage check finds a continuing loss of 3.5 psi in one minute. What does this mean?
Correct answer: It exceeds the applied-test limit and needs repair before driving
Secure the vehicle against rolling, charge to governor cut-out, shut off the engine and release the parking brakes. Fully apply and hold the foot brake for the timed check. More than 3 psi per minute is excessive for a single vehicle; this is not the brakes-released test.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Air Leakage Rate, printed 5-7 (physical PDF page 101) ↗
For a tractor and one air-braked semitrailer, what is the maximum one-minute loss in Pennsylvania's full-foot-brake leakage check?
Correct answer: 4 psi
Secure the combination against rolling and charge to governor cut-out. With the engine off, parking brakes released and trailer air supply open, fully apply and hold the foot brake for the timed check. More than 4 psi per minute is excessive. This is separate from the brakes-released test.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Air Leakage Rate, printed 5-7 (physical PDF page 101) ↗
In Pennsylvania CDL manual section 5.3.3, what loss rate should a single vehicle stay below during the brakes-released leakage check?
Correct answer: 2 psi in one minute
Secure the vehicle against rolling, fully charge the system, stop the engine and release the brakes. The chapter specifies a loss rate of less than 2 psi per minute for a single vehicle. This is the brakes-released test, not the applied test.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Air Leakage Rate, printed 5-7 (physical PDF page 101) ↗
In Pennsylvania's driver inspection procedure, which buildup is expected for a typical dual air system at operating engine rpm?
Correct answer: 85 to 100 psi within 45 seconds
PUB 223's driver check specifies 85 to 100 psi within 45 seconds at operating rpm. Larger-than-minimum tanks may require longer; consult the manufacturer. This driver inspection benchmark is not the separate federal new-vehicle certification test.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Check Rate of Air Pressure Buildup, printed 5-7 (physical PDF page 101) ↗
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
In a safe test area, check normal air pressure, release the parking brake, move forward at about 5 mph and apply the service brakes firmly. Check for pulling, an unusual feel or delayed stopping.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Service Brakes, printed 5-8 (physical PDF page 102) ↗
On a vehicle without ABS, what defines controlled braking during an emergency stop?
Correct answer: Brake as hard as possible without locking the wheels
For controlled braking, brake firmly without locking the wheels and keep steering movements small. If wheels lock or a larger steering correction is needed, release the brakes and reapply as soon as possible. This question concerns a vehicle without ABS.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.3 Emergency Stops — Controlled Braking, printed 5-9 (physical PDF page 103) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.4 Stopping Distance; total-distance formula, printed 5-9 (physical PDF page 103) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.4 Stopping Distance, printed 5-9 (physical PDF page 103) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.5 Brake Fading or Failure, printed 5-9 (physical PDF page 103) ↗
What should a driver do when the low-air warning activates on the road?
Correct answer: Stop and park safely as soon as possible
A warning may indicate a leak; stop safely before pressure falls far enough for spring brakes to apply.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.7 Low Air Pressure, printed 5-10 (physical PDF page 104) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.2 Dual Air Brake Systems, printed 5-6 (physical PDF page 100) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.2 Dual Air Brake Systems, printed 5-6 (physical PDF page 100) ↗
What should you inspect on a belt-driven air compressor during the engine-compartment check?
Correct answer: Belt condition and tightness
A loose or damaged drive belt can prevent correct air supply. Section 5.3.1 calls for checking the belt condition and tightness.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.1 During Step 2 Engine Compartment Checks, printed 5-6 (physical PDF page 100) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.2 During Step 5 Walkaround Inspection — automatic slack adjusters, printed 5-6 (physical PDF page 100) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.16 Antilock Braking Systems, printed 5-4 (physical PDF page 98) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.16 Antilock Braking Systems, printed 5-4 (physical PDF page 98) ↗
Brakes are very wet and temperatures are freezing. What parking precaution does Pennsylvania's air-brake chapter describe?
Correct answer: Use wheel chocks on a level surface instead of applying wet parking brakes
Very wet parking brakes may freeze in place. Secure the vehicle with wheel chocks on level ground; never leave it able to roll.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.8 Parking Brakes, printed 5-10 (physical PDF page 104) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.2 During Step 5 Walkaround Inspection — Check Brake Drums (or Discs), Linings, and Hoses, printed 5-7 (physical PDF page 101) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.13 Front Brake Limiting Valve, printed 5-3 (physical PDF page 97) ↗
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 damaged hose can lose air; a normal pressure reading does not make the defect acceptable.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.2 During Step 5 Walkaround Inspection — Check Brake Drums (or Discs), Linings, and Hoses, printed 5-7 (physical PDF page 101) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), Section 5 introduction, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.7 The Brake Pedal, printed 5-2 (physical PDF page 96) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.1 Air Compressor, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.5 Alcohol Evaporator, printed 5-2 (physical PDF page 96) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.6 Safety Valve, printed 5-2 (physical PDF page 96) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.8 Foundation Brakes — S-cam Brakes, printed 5-2 (physical PDF page 96) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.8 Foundation Brakes — Disc Brakes, printed 5-3 (physical PDF page 97) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.9 Supply Pressure Gauges; 5.1.10, printed 5-3 (physical PDF page 97) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.12 Stop Light Switch, printed 5-3 (physical PDF page 97) ↗
In a safe test area, how do you check whether the applied parking brake holds?
Correct answer: Apply it, then gently pull in low gear
Use a gentle holding test rather than forceful acceleration.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Parking Brake, printed 5-8 (physical PDF page 102) ↗
In Pennsylvania CDL manual section 5.3.3, what loss rate should an air-braked combination stay below with the brakes released?
Correct answer: 3 psi in one minute
Secure the combination against rolling, fully charge the system, stop the engine and release the brakes. The chapter specifies less than 3 psi per minute; the applied combination limit of 4 psi is a different test.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.3.3 Step 7 Final Air Brake Check — Test Air Leakage Rate, printed 5-7 (physical PDF page 101) ↗
A bus has a separate emergency air supply for releasing its spring brakes. Why must an emergency move be planned carefully?
Correct answer: The reserve may release the brakes only a few times
The separate reservoir is a limited emergency supply. Repeated releases may exhaust it and leave the bus stopped in a dangerous location.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.15 Parking Brake Controls — Dual Parking Control Valves, printed 5-4 (physical PDF page 98) ↗
Why may an automatic air-tank drain have an electric heater?
Correct answer: To help prevent the drain from freezing
An electric heating device helps prevent the automatic drain from freezing in cold weather.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.1.4 Air Tank Drains, printed 5-1 (physical PDF page 95) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.6 Proper Braking Technique, printed 5-10 (physical PDF page 104) ↗
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: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 5.4.5 Brake Fading or Failure, printed 5-9 (physical PDF page 103) ↗
