As technologically marvelous as modern trucks are, according to the Commercial Vehicle Safety Alliance, almost 20 percent of trucks placed out of service at roadside inspections last year were busted for problems in the humble foundation-brake system. And over half of those were cases of misadjustment.
How is that possible, when automatic slack adjusters have been mandatory on new vehicles for a decade?
The answer is that some maintenance managers take the “automatic” in ASA too literally. ASAs need inspection and maintenance like the rest of the brake system.
ASAs can fail to keep brakes in proper adjustment for many reasons, including misinstallation, improper initial adjustment and internal contamination. There even have been scattered reports from fleet members of the Technology & Maintenance Council (TMC) of new vehicles being delivered with misinstalled/misadjusted ASAs.
Moreover, ASAs sometimes appear to be at fault because of brake maintenance problems elsewhere. Things like deformed brackets, worn cam bushings, tired return springs and lack of lubrication can conspire to prevent an ASA from keeping tight reigns on brake adjustment.
Remember, these adjusting mechanisms perform within a tight window of tolerance, which corresponds to a running clearance of .020 in. to .030 in., depending on the brand. Since any slop will inhibit proper ASA operation, brake maintenance must be done to a high standard.
Slacks themselves are a good place to start inspecting. Look for deterioration or damage to boots or seals. Even a small amount of water sucked into an adjusting mechanism, especially in winter, contaminates lubricant and causes corrosion. It can freeze on tolerance-sensitive surfaces and inhibit or prevent adjustments until the ice thaws.
