393.53 Automatic brake adjusters and brake adjustment indicators.
(a) Automatic brake adjusters (hydraulic brake systems).
Each commercial motor vehicle manufactured on or after October 20, 1993, and equipped with a hydraulic brake system, shall meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard no. 105 (49 CFR 571.105, S5.1) applicable to the vehicle at the time it was manufactured.
(b) Automatic brake adjusters (air brake systems).
Each commercial vehicle manufactured on or after October 20, 1994, and equipped with an air brake system, shall meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard No. 121 (49 CFR 571.121, S5.1.8) applicable to the vehicle at the time it was manufactured.
(c) Brake adjustment indicator (air brake systems).
On each commercial motor vehicle manufactured on or after October 20, 1994, and equipped with an air brake system which contains an external pushrod, the condition of service brake under-adjustment shall be displayed by a brake adjustment indicator conforming to the requirements of Federal Motor Vehicle Safety Standard No. 121 (49 CFR 571.121, S5.1.8) applicable to the vehicle at the time it was manufactured.

571.121, subsection S5.1.8 (b)
Brake Indicator.
For each brake equipped with an external automatic adjustment mechanism and having an exposed pushrod, the condition of service brake under-adjustment shall be displayed by a brake adjustment indicator that is discernible when viewed with 20/40 vision from a location adjacent to or underneath the vehicle.