Last Thursday morning, I brought my Blackbird down off center stand in the garage to pack it for a cross country ride. To my surprise, the rear tire was flat. The tires, both of them, were holding proper pressure consistently when I last checked them before Thursday and was riding the beast, which was probably, oh, a month ago. I took the FJR on the trip instead; it never gives me any guff at all. Later that day, I called the shop to come get the Honda at my house and perhaps see if they could diagnose the problem then and there and let me know. They could not. They took it to the shop, pressurized the tire and did every leak and pressure test they know of to see what the problem could have been. It held pressure consistently and showed no sign of leakage or deflation. They have had the bike for a week now and the tire is holding pressure. The tires are new; no more than a few hundred miles on them. I went ahead and had them install some 90 degree valve stems and new valve cores just in case. What could have possibly happened to cause the unexplained deflation of the rear tire? There must be some reason. I am not smart or experienced enough to come up with it; all I can think of is a leak so slight as to be below any level of detection except with the passage of time, and one morning, she's flat. Thoughts?