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) ↗
