That for the establishment and maintenance of such guaranty fund, the board of commissioners or directors of any such district be and they hereby are authorized and directed annually to levy a tax, in addition to all other taxes, upon the real property within such district subject to general taxation for state and county purposes, of two mills for every dollar of the assessed valuation thereof, as shown by the last preceding assessment for state and county taxes; provided, however, that whenever and so long as the amount of such guaranty fund equals or exceeds an amount equal to fifteen per centum of the total bonded indebtedness of such district, no further levy shall be made therefor, unless and until the amount of such fund may be depleted by payments therefrom as hereinafter authorized.