Rates for services furnished by any such system, project or combined system shall be fixed prior to the issuance of the bonds. Such rates shall be sufficient to provide for the payment of the interest upon and the principal of all such bonds as and when they become due and payable, to create a bond and interest redemption fund therefor, to provide for the payment of the expenses of administration and operation and such expenses for maintenance of the system, project or combined system as are necessary to preserve it in good repair and working order, to build up a reserve for depreciation of the existing system, project or combined system and to build up a reserve for improvements, betterments and extensions to the existing system, project or combined system other than those necessary to maintain it in good repair and working order as herein provided. Such rates shall be fixed and revised from time to time so as to produce these amounts and the governing body shall covenant and agree in the ordinance authorizing the issuance of the bonds and on the face of each bond at all times to maintain such rates for services furnished by such system, project or combined system as shall be sufficient to provide for the foregoing.