Michigan Compiled Laws, Chapter 257. Motor Vehicles § 257.1813

Sec. 13. (1) A school bus that is equipped with an air brake system shall be equipped with an emergency stopping system designed and constructed to permit modulated control of brake application and release and to prevent release by the driver unless energy is available for reapplication. The air brake system shall be provided with an electrically heated moisture ejector on each air reservoir or shall be provided with an after cooler type air dryer with an electrically heated purge valve. The use of moisture ejectors together with an air dryer is optional. Each air reservoir in the system shall be equipped with a manual petcock valve on the bottom of the tank. If the automatic moisture ejection valve is equipped with a manual drain control or built-in petcock, a separate manual petcock is not required. The air reservoir tanks shall be mounted on the chassis frame with not less than 15 inches of clearance between the ground and the lowest part of the tank or tank component.

(2) A school bus manufactured after July 15, 1982 that has a rated seating capacity of 60 or more passengers is not required to be equipped with an air brake system.

(3) A school bus manufactured before July 7, 1981 may use an air dryer system.