Whenever the governing body of a sewer district has apportioned the cost of improvements on the basis of the assessed valuation of the property in the district and if at any time there is a deficit in any bond and interest fund of the sewer district, the governing body of the district shall make a levy based on the assessed valuation of all land and improvements in the main sewer district necessary to raise the required funds to pay the deficit and the principal of the bonds and interest in full.