At or before the time of the issuance of any bonds under this article the board shall by resolution or in the trust indenture provide for the creation of a sinking fund and for monthly payments into such fund from the revenues of the public service properties operated by the district such sums in excess of the cost of maintenance and operation of such properties as will be sufficient to pay the accruing interest and retire the bonds at or before the time each will respectively become due and to establish and maintain reserves therefor. All sums which are or should be, in accordance with such provisions, paid into such sinking fund shall be used solely for payment of interest and for the retirement of such bonds at or prior to maturity as may be provided or required by such resolutions.