The said board shall establish such water, electric, and gas rates as will at all times ensure to the city a proper income, sufficient if possible, to pay the interest and to provide a fund to pay the principal of all bonds to be issued, or heretofore issued, in connection therewith as well as to pay all the expenses and costs of maintaining and keeping in repair said works; provided, however, that such rates shall not be exorbitant or unreasonable, or so as to prevent the proper and extended use thereof; provided, also, that the present established rates shall continue until modified, changed or discontinued by act of the board.