Pushrod Stroke Out of Limit on One Side
During a pre-trip inspection of a 2016 tandem-axle dump truck, the driver measures the left rear service chamber pushrod. With the brakes released, the pushrod is at 0.5 inch; with the brake pedal fully applied and the system at 110 psi, it extends to 2.75 inches. The right rear chamber extends from 0.5 to 1.75 inches. The left slack adjuster is an automatic type and the adjusting pawl moves freely by hand. The chamber is a type 30/30 with a stamped maximum stroke of 2.0 inches. The driver has a 25-mile trip with a 12-ton load and a 4-mile downgrade. The truck passed a leak-down check with 2 psi loss in one minute. There is no obvious air leak and the brake linings look thick from the outside. The driver has a basic tool kit and wants to adjust the slack adjuster to get the truck to the job site.
1. Is the left rear brake within adjustment limits? What does the side-to-side difference suggest?
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No. The left chamber stroke is 2.75 inches, which exceeds the chamber's stamped maximum of 2.0 inches and is well beyond normal adjustment. The right side at 1.75 inches is within the stamped limit, but the left is overstoked. A side-to-side difference of about 1 inch at full application suggests the left brake is not transferring force correctly. Possible causes:
- a defective or slipping automatic slack adjuster - the pawl moving freely by hand is not proof it is adjusting under load;
- a worn or broken chamber spring, bent pushrod, or loose chamber mounting;
- an incorrectly installed slack adjuster or a foundation brake problem such as worn shoes, drum, or cam;
- a leak or restriction that prevents full chamber pressure, though the 2 psi leak-down makes a major leak less likely.
The correct conclusion is that the left rear brake is out of adjustment and the cause is not simply normal wear.
2. Should the driver adjust the automatic slack adjuster and make the run? What inspection and repair steps are appropriate?
Read the worked answer
No. Manually adjusting an automatic slack adjuster to shorten the stroke can mask the real defect. Automatic slack adjusters are designed to maintain adjustment during normal service; if one is out of adjustment, the usual cause is a foundation brake problem, a worn or broken component, or a failed adjuster. Adjusting it by hand may make the stroke look acceptable while the brake still does not apply properly, or it may cause drag and overheating. Appropriate steps:
- Chock the wheels and secure the truck.
- Compare both sides and inspect the left chamber mounting, pushrod, clevis pin, slack adjuster, cam, shoes, and drum.
- Check for a broken chamber spring or a chamber that is not returning.
- Replace or repair the defective component; if it is a manual slack adjuster, adjust it per the manufacturer's procedure, but this is an automatic type.
- Re-measure stroke at full pressure and confirm it is within the stamped limit before moving.
With a 12-ton load and a 4-mile downgrade, an overstoked or dragging rear brake is an out-of-service condition. A basic tool kit and a desire to reach the job site do not make it safe.