Backflow preventer should be tested and repaired if necessary by a certified inspector each year. It is required in my city under penalty of disconnect of water service. Regardless of the legal requirement which may or may not be enforced where you are, it is the wise thing to do. Inspection cost me $35 per year and the occasional repairs are quite inexpensive when need. I think you might be wise to consider having your device repaired and tested by a certified inspector. It is not a complex job, but it does require special equipment as well as know how. The purpose of these devices is to prevent contamination of the water supply, so it should not be done by guess work.