Slack Adjuster Inspection: Push-Pull Test
Slack adjusters are mechanical linkages that connect the brake chamber pushrod to the S-cam in drum brake systems. Their function is to compensate for brake lining wear by maintaining the correct stroke length of the brake chamber pushrod. A properly adjusted slack adjuster ensures that the brakes apply effectively within the designed pushrod stroke range. The pre-trip push-pull test for manual slack adjusters: with the parking brakes fully released (air pressure at normal operating level — 100-120 PSI) and the wheels chocked, grasp each slack adjuster and pull it firmly toward you. Maximum allowable movement: 1 inch (on a 5/8-inch diameter pushrod at 90 degrees to the adjuster). Any slack adjuster that moves more than 1 inch is likely out of adjustment. If adjusters on a tandem axle group have significantly different travel (one moves ¼ inch, one moves ¾ inch), this indicates unequal brake adjustment that will cause uneven braking — a safety issue and an out-of-service violation. Automatic slack adjusters (ASAs): most modern vehicles use ASAs that self-adjust with each brake application. ASAs should be inspected but not manually adjusted unless trained to do so — improper manual adjustment of an ASA can damage the mechanism. If an ASA consistently reads out of adjustment, the ASA itself may be defective and needs replacement.