CDL Air Brake Brake Adjustment and Maintenance 1 — Questions and Answers
Question 1: What is the purpose of a slack adjuster in an air brake system?
- To adjust air pressure delivered to the brake chambers
- To maintain proper brake shoe-to-drum clearance as brake linings wear (Correct answer)
- To regulate the governor cut-out pressure
- To control air flow to the spring brakes
Correct answer: To maintain proper brake shoe-to-drum clearance as brake linings wear
Slack adjusters maintain proper clearance between brake shoes and drum as lining wears down, ensuring the push rod does not have to travel too far.
Question 2: What is the maximum push rod stroke allowed before brakes are considered out of adjustment?
- Whatever the driver feels is sufficient based on pedal feedback
- Varies by brake chamber size, but typically 1¾ to 2 inches maximum (Correct answer)
- 3 inches maximum for all brake chamber sizes
- 4 inches maximum for all brake chamber sizes
Correct answer: Varies by brake chamber size, but typically 1¾ to 2 inches maximum
Maximum push rod stroke limits vary by brake chamber size but are typically around 1¾ to 2 inches — exceeding this means brakes are out of adjustment.
Question 3: What is an automatic slack adjuster (ASA)?
- A device that automatically adjusts the governor cut-out pressure
- A self-adjusting mechanism that maintains proper brake adjustment without manual intervention (Correct answer)
- An ABS component that adjusts brake pressure during skids
- A computer-controlled system that monitors brake temperature
Correct answer: A self-adjusting mechanism that maintains proper brake adjustment without manual intervention
An automatic slack adjuster continuously self-adjusts to maintain proper brake lining-to-drum clearance without requiring manual adjustment.
Question 4: Even with automatic slack adjusters installed, what must a driver do during pre-trip inspection?
- Nothing — automatic adjusters require no inspection whatsoever
- Manually adjust each slack adjuster to the desired setting
- Visually inspect and check push rod stroke to verify proper adjustment (Correct answer)
- Replace all brake linings at each inspection interval
Correct answer: Visually inspect and check push rod stroke to verify proper adjustment
Drivers must still visually inspect slack adjusters and verify push rod stroke during pre-trip inspections, as automatic adjusters can still malfunction.
Question 5: What is the minimum brake lining thickness allowed on drum brakes before replacement is required?
- ¼ inch for drum brakes (Correct answer)
- ⅛ inch for drum brakes
- ¾ inch for drum brakes
- There is no minimum — replace only when brakes stop working
Correct answer: ¼ inch for drum brakes
Brake linings must be replaced when they reach ¼ inch thickness on drum brakes to maintain safe stopping performance and prevent metal-to-metal contact.
Question 6: What condition requires immediate brake lining replacement regardless of lining thickness?
- Lining has been in service for more than 6 months
- Lining is contaminated with oil or grease (Correct answer)
- Lining color has changed from its original appearance
- Vehicle has traveled more than 50,000 miles since last replacement
Correct answer: Lining is contaminated with oil or grease
Oil or grease contamination on brake linings significantly reduces friction, making them ineffective and requiring immediate replacement.
CDL Air Brake Brake Adjustment and Maintenance 1
What is the purpose of a slack adjuster in an air brake system?