Michigan Compiled Laws, Chapter 339. Occupational Code § 339.6119

Sec. 1119.  (1) An individual who is licensed as a master plumber may request that the master plumber license be retained by the department as an inactive license for a period that does not exceed 3 years.

(2) The department shall issue an inactive master plumber license as an active license if requested by the licensee, the licensee pays the relicensure fee described in article 4   1 at the time of the request, the licensee holds a journey plumber license, and the licensee has renewed his or her journey plumber license each year since the license has been inactive.

(3) An individual shall not simultaneously hold an active master plumber license and journey plumber license.  An individual who holds an active master plumber license may work as a journey plumber.