The tax which is levied in connection with the original bond issue shall remain in force for that purpose until a new levy is made. The board may, from time to time, increase or diminish the tax for the purpose of adjusting the tax to the taxable values of taxable property in the district and the amount to be collected, and the increase or decrease in the tax shall be sufficient to provide enough money in the interest and sinking fund to make annual payments on outstanding bonds.