After 420 hrs I noticed some air in the brake line (although there is no performance change). Since the air are upstream from the pilot side master cylinders, brake performance has not been affected. Today, during condition inspection, I decided to bleed the brakes. See more details at this page. After I bleed the brakes segments of the tube still looks like to have air. After careful inspection, it really was due to the light reflection. I did not have air in the tube after all. However, my reservoir is a little low. I don't regret the trouble. At least it assure me my brake lines are fine.