Whenever bonds are issued as provided by this Division 71, it shall be the duty of the corporate authorities to establish charges and fees for the use of any such parking facilities sufficient at all times to pay maintenance and operation costs, and principal of and interest upon such bonds, and all revenues derived from the operation thereof shall be set aside as a separate fund and account and used only as hereinafter provided, except as otherwise provided in paragraph (c) of Section 11-71-1.
65 ILCS 5/11-71-4