40 ILCS 5/11-183
A vacancy in the membership of the board shall be filled as follows:
If the vacancy is that of an appointive member, the person or body who appointed the member shall appoint a person to serve for the unexpired term. If the vacancy is that of an elective member, the remaining members of the board shall appoint a successor, who shall be an employee or annuitant (as the case may be) who is qualified to hold the position, to serve during the remainder of the unexpired term.
Any appointive or elective member who leaves the service of the city, other than the annuitant member, shall automatically cease to be a member of the board. If the annuitant member ceases to be an annuitant of the Fund, he or she shall cease to be a member of the board and the position shall be deemed to have become vacant.
40 ILCS 5/11-183