CDL Air Brake Brake Adjustment and Maintenance 2 — Questions and Answers
Question 1: What is the purpose of the brake drum in an S-cam foundation brake system?
- To store compressed air for emergency braking
- To provide a friction surface for brake shoes to press against to slow the wheel (Correct answer)
- To house the air brake chamber assembly
- To connect the axle shaft to the wheel hub
Correct answer: To provide a friction surface for brake shoes to press against to slow the wheel
The brake drum rotates with the wheel and provides the friction surface that brake shoes press against to convert kinetic energy into heat and slow the vehicle.
Question 2: When must a brake drum be removed from service and replaced?
- After any visible surface rust appears
- When worn to the maximum diameter specification marked on the drum (Correct answer)
- After exactly 100,000 miles of service
- Whenever a new set of brake shoes is installed
Correct answer: When worn to the maximum diameter specification marked on the drum
Brake drums have a maximum diameter specification stamped on them — exceeding this reduces wall thickness and can lead to failure under braking stress.
Question 3: Why is a cracked brake drum considered a serious safety hazard?
- It creates minor surface wear marks that reduce friction
- It can fail catastrophically under the stress of braking, causing complete loss of braking on that wheel (Correct answer)
- It has simply been resurfaced too many times
- It develops minor rust pitting that reduces efficiency slightly
Correct answer: It can fail catastrophically under the stress of braking, causing complete loss of braking on that wheel
A cracked brake drum can fail catastrophically under braking stress, potentially causing complete and sudden loss of braking on that wheel.
Question 4: What does it indicate if a vehicle pulls noticeably to one side when the brakes are applied?
- This is normal for vehicles with different tire pressures on each side
- One or more brakes on one side may be out of adjustment, worn, or defective (Correct answer)
- The steering alignment needs adjustment unrelated to the brakes
- The vehicle is overloaded on the side it is pulling toward
Correct answer: One or more brakes on one side may be out of adjustment, worn, or defective
Pulling to one side when braking indicates unequal braking force, typically caused by a brake out of adjustment, contaminated lining, or a defective component.
Question 5: How should a driver verify brake adjustment during a pre-trip inspection?
- Apply brakes at highway speed and check the resulting stopping distance
- With wheels chocked and 90 psi in the system, measure push rod stroke with brakes applied (Correct answer)
- Listen for squealing sounds during a low-speed test stop
- Review the maintenance logbook and rely on shop records only
Correct answer: With wheels chocked and 90 psi in the system, measure push rod stroke with brakes applied
With wheels chocked and air pressure at 90 psi, push rod stroke must be measured when brakes are applied and must not exceed the maximum for the chamber size.
Question 6: What is a 'brake spider' in an S-cam drum brake system?
- A type of automatic slack adjuster mechanism
- The fixed mounting bracket attached to the axle that supports brake shoes and foundation brake components (Correct answer)
- The S-cam shaft bearing assembly
- A protective cover installed over the brake chamber
Correct answer: The fixed mounting bracket attached to the axle that supports brake shoes and foundation brake components
The brake spider is the fixed mounting bracket attached to the axle flange that supports the brake shoes, anchor pins, and other foundation brake components.
CDL Air Brake Brake Adjustment and Maintenance 2
What is the purpose of the brake drum in an S-cam foundation brake system?