Excessive Slack Adjuster Free Play on an Automatic System
During a pre-trip at a shipper's yard, you inspect a 2018 Kenworth T680 tractor with a 53-foot van trailer. The tractor has automatic slack adjusters. With the parking brakes released and air pressure at 110 psi, you pull each slack adjuster by hand. Three move about 1/4 to 1/2 inch. The left rear drive-axle slack adjuster moves about 1.5 inches before you feel resistance. The chamber clamp band is dry, and the pushrod boot is torn. You apply the brakes and watch the pushrod extend, but you do not have the chamber's published stroke limit with you. The trailer is loaded to 42,000 lb. You are 25 miles from the terminal, and dispatch says the load must leave in 40 minutes. Another driver says, 'Just crank the automatic slack down; they all get loose.' You have hand tools but no replacement slack adjuster.
1. What does the 1.5-inch hand movement indicate, and is it acceptable?
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With brakes released, a slack adjuster should not move more than about 1 inch by hand. Three adjusters moving 1/4 to 1/2 inch are within that check, but the left rear drive-axle unit moves about 1.5 inches, which is excessive free play. That can mean the slack adjuster is worn or loose on its mounting, the cam or bushings are worn, the chamber pushrod is misadjusted, or the foundation brake has another mechanical problem. The torn pushrod boot and dry clamp band are additional warning signs because they let water and dirt into the chamber. You should also measure applied pushrod travel at 90 psi against the chamber's published limit, but the failed hand check is already enough to reject the brake.
2. Should you manually adjust the automatic slack adjuster to remove the play? Explain why or why not.
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No. Automatic slack adjusters are designed to adjust during normal brake applications. Manually cranking one down to hide excessive free play can mask a worn or damaged component, and it may prevent the automatic mechanism from working correctly. It can also set the brake too tight, causing drag and heat, or leave the pushrod in a position that leads to overstroke and a partial loss of braking. A properly functioning automatic slack should not need routine manual adjustment. If it has excessive free play, the correct response is to find out why and repair or replace the affected part, not to crank it down at the yard.
3. What is the safe decision for this load and truck, and what should you document?
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Do not dispatch the truck. A 42,000 lb trailer load needs all tractor brakes working correctly, and one drive-axle brake with 1.5 inches of free play is a defect. The 40-minute schedule does not change the brake system's condition. Notify the shop and dispatch, record the excessive free play and the torn boot on the DVIR, and have the brake inspected and repaired or use another tractor. If a mechanic finds a failed automatic slack adjuster, replace it rather than manually adjusting it. After repair, recheck free play and applied pushrod travel before the truck leaves.