If the petition for formation of the district provides therefor or the owners of real property in the district agree thereto, a district shall not cease to exist upon the acquiring, construction, or completion of the improvement, but it shall continue to exist for the purpose of preserving, maintaining, and operating the improvement, replacing equipment, paying salaries to employees, and performing any other functions or services authorized in this chapter. To this end, the board may, from time to time, make such additional levies based upon the assessment of benefits as may be necessary for such purposes. However, the amount of the total levies shall not exceed the assessed benefits and interest thereon.