Suburban improvement districts shall not cease to exist upon the acquiring, construction, or completion of the improvement but 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 subchapter. To this end, the board of commissioners may, from time to time, make such additional levies based upon the assessment of benefits as may be necessary for these purposes. However, the amount of the total levies shall not exceed the assessed benefits and interest thereon.