If the interest and principal of the bonds and all charges to protect or secure them are paid when due an amount for the necessary and reasonable maintenance and operation costs of the enterprise, which costs include the reasonable expenses of management, repair and other expenses necessary to maintain and preserve the enterprise in good repair and working order, may be apportioned from the revenues, and subject to any limiting covenants in the resolution providing for the issuance of bonds, the remaining surplus may be used for any lawful purpose of the local agency.