Definition
On an air drum brake, air pressure in the brake chamber pushes a pushrod. The pushrod turns the slack adjuster, which rotates the camshaft and forces the shoes against the drum. As linings and drums wear, the pushrod has to travel farther. The slack adjuster takes up that extra travel, or slack, so the brake still applies within its working stroke.
Federal requirements
- Automatic adjusters: 49 CFR 393.53(b) requires each commercial motor vehicle manufactured on or after October 20, 1994 and equipped with air brakes to meet the automatic brake adjustment requirements of FMVSS No. 121. Hydraulic-braked vehicles built on or after October 20, 1993 have a parallel requirement under FMVSS No. 105.
- Adjustment indicator: on air-braked vehicles built on or after October 20, 1994 with an external automatic adjuster and an exposed pushrod, under-adjustment must be shown by a brake adjustment indicator (393.53(c)).
- Stroke limits: the maximum pushrod stroke must not exceed the readjustment limits in 393.47(e) and Appendix A to Part 396, which vary by chamber type and size. For annual inspections, stroke is measured with the engine off and reservoir pressure of 80 to 90 psi with brakes fully applied.
Shop practice
Automatic slack adjusters should not need routine manual adjustment. A brake that keeps going out of stroke points to a problem, such as a worn adjuster, worn cam bushings or other foundation brake wear, rather than a reason to keep backing off the adjuster. Fix the cause, then verify stroke.
Brake work under federal rules must be done or overseen by someone qualified under 49 CFR 396.25; see brake inspector. See also brake chamber and air brake.