The board of directors of any district securing or to secure any loan hereunder shall have power and authority to pledge and/or mortgage the income, assets and property of said district, and are authorized and empowered, and it shall be the duty of said board, to make special levy of assessment and assessments, from year to year, against all lands within the district to meet the payment and payments, including interest and sinking fund, obligated under any loan made hereunder; that such special assessments shall be increased not less than 15 per centum over and above the amount and amounts deemed to be required to meet the loan obligation, and if the sum realized from such special assessment and assessments be insufficient then any and all other income and property of the district shall be used as necessary to meet such obligation, except that no funds of the district shall be used to pay obligations hereunder where such funds have been made available to pay the outstanding bonds of such district, or interest thereon, or other obligations of the district shown of the public records of the county clerk of the county in which said district be situated, or if situated in more than one county, then the said records of each county.