S5.5.1. Activation. An indicator shall be activated when the ignition switch is in the on (run) position and whenever any of conditions (a) through (g) occur:
(a) A gross loss of fluid or fluid pressure (such as caused by rupture of a brake line but not by a structural failure of a housing that is common to two or more subsystems) as indicated by one of the following conditions (chosen at the option of the manufacturer):
(1) A drop in the level of the brake fluid in any master cylinder reservoir compartment to less than the recommended safe level specified by the manufacturer or to one-fourth of the fluid capacity of that reservoir compartment, whichever is greater.
(2) For vehicles equipped with a split service brake system, a differential pressure of 1.5 MPa (218 psi) between the intact and failed brake subsystems measured at a master cylinder outlet or a slave cylinder outlet.
(3) A drop in the supply pressure in a brake power unit to one-half of the normal system pressure.
(b) Any electrical functional failure in an antilock or variable brake proportioning system.
(c) Application of the parking brake.
(d) Brake lining wear-out, if the manufacturer has elected to use an electrical device to provide an optical warning to meet the requirements of S5.1.2(a).
(e) For a vehicle with electrically-actuated service brakes, failure of the source of electric power to those brakes, or diminution of state of charge of the batteries to less than a level specified by the manufacturer for the purpose of warning a driver of degraded brake performance.
(f) For a vehicle with electric transmission of the service brake control signal, failure of a brake control circuit.
(g) For an EV with a regenerative braking system that is part of the service brake system, failure of the RBS.


49 CFR § 571.135

Scoping language

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