Air Brake Test Adjustment & Testing 2 — Questions and Answers
Question 1: What is the purpose of the automatic slack adjuster on air brake systems?
- To maintain proper brake lining-to-drum clearance automatically (Correct answer)
- To increase air pressure in the system
- To activate the spring brakes when air is lost
- To regulate the governor cut-in pressure
Correct answer: To maintain proper brake lining-to-drum clearance automatically
Automatic slack adjusters maintain the correct clearance between the brake lining and drum as linings wear, eliminating the need for manual adjustment during normal operation.
As brake linings wear down over time, the clearance between the lining and the drum increases, which reduces braking efficiency. Automatic slack adjusters continuously compensate for this wear by taking up the extra slack. While they adjust automatically during normal use, they should still be checked during pre-trip inspections for proper operation.
Question 2: When manually adjusting brake slack, which direction do you turn the adjusting bolt to tighten (reduce slack)?
- Clockwise (Correct answer)
- Counter-clockwise
- Either direction depending on axle side
- Slack adjusters are never manually turned
Correct answer: Clockwise
Turning the adjusting bolt clockwise tightens the brake by moving the brake shoe closer to the drum, reducing the pushrod stroke needed to apply the brake.
On most slack adjusters, turning the adjusting bolt clockwise (righty-tighty) moves the brake shoes closer to the drum, reducing the free stroke of the brake chamber pushrod. Drivers and mechanics must be careful not to over-adjust, as too little clearance causes brakes to drag, generating heat and accelerating lining wear.
Question 3: What is the maximum allowable brake chamber pushrod stroke for a standard type-30 chamber?
- 2 inches
- 3 inches (Correct answer)
- 1.5 inches
- 2.5 inches
Correct answer: 3 inches
A type-30 brake chamber has a maximum allowable stroke of 3 inches before the brakes are considered out of adjustment and the vehicle must be placed out of service.
Federal Motor Carrier Safety Administration (FMCSA) regulations specify maximum brake chamber stroke limits by chamber type. A type-30 chamber may not exceed 3 inches of pushrod stroke at the time of brake application. Exceeding this limit indicates the brakes are out of adjustment and will reduce stopping effectiveness significantly.
Question 4: During a brake adjustment check, you measure the pushrod stroke with the brakes applied at approximately what air pressure?
- 90 psi (Correct answer)
- 100 psi
- 60 psi
- 120 psi
Correct answer: 90 psi
Pushrod stroke is measured with full service brake application at approximately 90 psi to get a consistent and accurate reading of the slack condition.
When checking pushrod stroke, the brakes should be applied with approximately 90 psi of air pressure to ensure the brakes are fully applied. Measuring at a consistent pressure ensures accurate comparison to the maximum allowable stroke. This is typically done during a pre-trip inspection or as part of a brake inspection procedure.
Question 5: What does it mean if the low air pressure warning activates while checking brake adjustment during a pre-trip?
- The air compressor or governor may not be functioning properly (Correct answer)
- Brake lining thickness is at the minimum
- The spring brakes need adjustment
- Slack adjusters are over-tightened
Correct answer: The air compressor or governor may not be functioning properly
If the low air pressure warning activates during testing, it signals that the compressor is not maintaining adequate system pressure, pointing to a problem with the compressor or governor.
During pre-trip brake adjustment tests, the air compressor should rebuild pressure quickly. If the warning buzzer or light activates and pressure does not rise back above the warning threshold (usually 60 psi), it indicates the compressor, governor, or air system has a fault. This must be corrected before the vehicle is placed into service.
Question 6: Which of the following correctly describes the 'applied stroke' measurement used in brake adjustment checks?
- The distance the pushrod travels from rest when brakes are fully applied (Correct answer)
- The total length of the pushrod when the brakes are released
- The pressure required to move the pushrod 1 inch
- The distance between the slack adjuster arm and the brake chamber
Correct answer: The distance the pushrod travels from rest when brakes are fully applied
Applied stroke is the distance the brake chamber pushrod moves from its rest (released) position to its applied position when the service brakes are fully applied.
Applied stroke is measured by marking the pushrod position with the brakes released, then fully applying the brakes and measuring how far the rod has extended. If this distance exceeds the legal limit for that chamber type, the brakes are out of adjustment. Proper applied stroke ensures the brakes generate sufficient clamping force at the drum.
Air Brake Test Adjustment & Testing 2
What is the purpose of the automatic slack adjuster on air brake systems?