i have had this problem a few times with my f16 (which is now 7 years old) i discard the o ring. thoughroughly clean the hub, particularly the groove where the o ring sits.from hardwood or ply make a tool that is nice and flat, with a small groove that aproximatly mirrors the o ring groove, perhaps a little larger.insert silicon into the hub groove, and make sure you work it in. follow the groove with your ply tool, then remove all excess silicon (turps). assemble th other hub half, insert and screw in the hub attatchment bolts while the silicon is still wet to flatten the silicon but don't tighten, just make sure there is about 1 to 1.5 turns of the bolts left, let it dry for 24 to 48 hrs then tighten and fill with air. mine have lasted a year and i cant remember the last time i put air in them. and just you wait until the next test!