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) ↗
