CDL Air Brakes TestAir Brake System PartsMedium
During a pre-trip inspection, a driver notices that when the service brakes are applied, the pushrod on one of the brake chambers moves less than 1/2 inch. What does this likely indicate?
- AThe slack adjuster is seized and not allowing proper brake application.
- BThe brake is severely out of adjustment and requires immediate attention.
- CThe brake is within normal operating limits for adjustment.
- DThe air chamber diaphragm is ruptured, causing a loss of pressure.
Show answer & explanationAnswer & explanation
Correct answer: C. The brake is within normal operating limits for adjustment.
Properly adjusted air brakes typically have a pushrod stroke of 1/2 inch to 2 inches, depending on the brake chamber type. A movement of less than 1/2 inch indicates that the brake is likely in good adjustment, as excessive stroke (over 2 inches) is the concern for out-of-adjustment brakes.
Why the other options are wrong
- A. A seized slack adjuster would typically result in either no movement or excessive movement if it's stuck in an over-adjusted position, or inability to adjust.
- B. A stroke of less than 1/2 inch is usually good; excessive stroke indicates maladjustment.
- D. A ruptured diaphragm would cause a significant air leak and little to no pushrod movement, but the question implies some movement, and less than 1/2 inch is not indicative of rupture.
Slack Adjuster Pushrod Stroke
The distance the pushrod moves out of the brake chamber when the service brakes are fully applied, used to check brake adjustment.
- Should be between 1/2 inch and 2 inches (depending on chamber size/type).
- Excessive stroke (over 2 inches) indicates brakes are out of adjustment.
- Automatic slack adjusters should maintain proper stroke.
- Manual slack adjusters require periodic adjustment.
Memory trick: Pushrod stroke, a measured poke, tells if your brakes are fit for the road.