To levy and collect taxes as herein provided, the board shall in each year determine the amount of money necessary to be raised by taxation, and shall fix a rate of levy which, when levied upon every dollar of the taxable tangible property within the district as shown by the last completed assessment, and with other revenues, will raise the amount required by the district annually to supply funds for paying the expenses of organization and the costs of acquiring, supplying and maintaining the property, works and equipment of the district, which rate of levy shall not exceed fifteen cents on the one hundred dollars valuation unless approved by a vote of a majority of the voters of the district voting as provided herein, and which, in any event, shall not exceed twenty-five cents on the one hundred dollars valuation; and in addition thereto, shall make a levy to promptly pay in full when due all interest on and principal of general obligations of the district; and in the event of accruing defaults or deficiencies, an additional levy may be made as provided herein.