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) ↗
