Air Brake Test Air Brake Maintenance & Troubleshooting 1 — Questions and Answers
Question 1: When should slack adjusters on air brakes be checked for proper adjustment?
- Monthly as part of a routine maintenance schedule
- Only during annual DOT inspections
- At every pre-trip inspection before operating the vehicle (Correct answer)
- Weekly during the operator's regular walk-around
Correct answer: At every pre-trip inspection before operating the vehicle
Slack adjusters must be checked at every pre-trip inspection because out-of-adjustment brakes are a leading cause of brake failure.
Question 2: What is the purpose of an automatic slack adjuster on an air brake system?
- To increase air pressure delivered to the brake chambers when linings wear
- To automatically maintain the correct clearance between brake linings and drums as linings wear (Correct answer)
- To adjust air tank pressure when the compressor governor cycles off
- To prevent the parking brake from releasing accidentally while parked
Correct answer: To automatically maintain the correct clearance between brake linings and drums as linings wear
Automatic slack adjusters compensate for lining wear by continuously maintaining the correct stroke length for full braking force.
Question 3: An automatic slack adjuster that requires frequent manual readjustment most likely indicates what condition?
- Normal brake wear that simply requires regular operator attention
- A defective slack adjuster that should be replaced by a mechanic (Correct answer)
- Excessively high air pressure that is stretching the adjuster mechanism
- Brake drums that are worn oversize and need to be replaced
Correct answer: A defective slack adjuster that should be replaced by a mechanic
An automatic slack adjuster that cannot maintain its own adjustment is defective and must be replaced — not repeatedly readjusted.
Question 4: How do you manually check a slack adjuster for proper adjustment during a pre-trip inspection?
- Apply the parking brake and attempt to rotate each slack adjuster by hand
- With brakes fully released, push and pull each slack adjuster — movement should be less than 1 inch (Correct answer)
- Apply full service brake pressure and measure the temperature of each drum
- Observe the angle indicator markings with full service brakes applied at 90 psi
Correct answer: With brakes fully released, push and pull each slack adjuster — movement should be less than 1 inch
Slack adjusters should move less than 1 inch when pushed or pulled by hand with the brakes released — more movement indicates the brakes are out of adjustment.
Question 5: At what minimum brake lining thickness must drum brake linings be replaced under federal regulations?
- 1/4 inch (6.4 mm) at the thinnest point (Correct answer)
- 1/2 inch (12.7 mm) measured at the center of the shoe
- 1/8 inch (3.2 mm) anywhere on the lining surface
- Any thickness that passes a qualified mechanic's visual inspection
Correct answer: 1/4 inch (6.4 mm) at the thinnest point
FMCSA regulations require brake linings worn to 1/4 inch or less to be replaced before the vehicle is operated.
Question 6: What condition does a cracked brake drum represent under federal out-of-service criteria?
- Normal aging that does not significantly affect braking performance
- An out-of-service defect that prohibits operation until the drum is replaced (Correct answer)
- Minor surface cracking that can be addressed by resurfacing the drum
- A condition requiring increased monitoring but not immediate repair
Correct answer: An out-of-service defect that prohibits operation until the drum is replaced
A cracked brake drum is a federal out-of-service condition — the vehicle must be removed from service until the drum is replaced.
Air Brake Test Air Brake Maintenance & Troubleshooting 1
When should slack adjusters on air brakes be checked for proper adjustment?