This is becoming an increasingly common issue amongst kites of this vintage.
There appears to have been a large lot of bladders to have come out a common manufacturing facility where the valves were not properly attached to the latex bladder. I've seen a dozen myself in the last 12 months. Over time, when exposed to heat and with usage, they valve just peels off the latex.
My 06 Slingshot Fuel has had 6 of the valves peel away in the last few months.
Having seen a number of these abruptly let go and dump all the air from the leading edge, it's NOT a good idea to keep kiting on these til you fix them or you'll soon find yourself 1km out to sea with a deflated kite.
There is another thread that's been running for a bit that addresses how to fix these. I'd advise against the PVC plumbers' glue, but have had good success with the stick-on valves you can get from your local kite shops as well as using contact cement and adding another layer of bladder materials OVER the valve after you've put it back on with contact cement to ensure there's no chance of the air getting out.
The other thread is at:
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=46385