I read at least one documented case where a flop tube had become stiff over time. In other words, it didn't "flop" anymore. If that wasn't bad enough, it had become stiff enough that it didn't flop all the way down in the standard aircraft upright orientation, meaning that the pilot didn't have access to all the fuel in that tank during normal upright flight. That can lead to fuel starvation when you still think you have plenty of fuel. If you install any flop tubes, I would think you would want to check for this on your annual inspection. Maybe someone knows the service life of a flop tube, and how often they need to be replaced. I'm no expert on this. If I can remember where I saw that story, I'll post it here. Maybe someone else can remember seeing it, too. It was a long time ago when I was building my wings and considering this question. I ended up putting two standard pickups in my tanks.