SDCL § 32-18-23: Combined air and vacuum power warning signal devices--Gauge not considered adequate.

When a vehicle required to be equipped with a warning device is equipped with both air and vacuum power for the operation of its own brakes or the brakes on a towed vehicle, the warning devices required by §§ 32-18-20 and 32-18-22 may be, but are not required to be, combined into a single device which will serve both purposes. A gauge or gauges indicating pressure or vacuum shall not be deemed to be an adequate means of satisfying this requirement.