Thanks for the answers guys. You do exactly as I did. Let me say it was easier to pack with the old double valve...
The best solution are two different valves, one for inflating and another one (with no membrane) for deflating, as seen on many other brands kite. The new Core one works but is definitively not the most convenient I've ever tried
I echo jumarcil's response on this one.
I usually leave my struts inflated 90% of the time, roll the kites up and store in long rip-stop sleeves and keep them in my van. When I deflate I usually insert the stick, roll one end into the middle, then roll up my lines while the remaining air is exhausted, and then roll the other end into the middle. (I've done this with all kites in the past). I press down on the LE a little as I get to the middle strut to get all the air out, but there is not much resistance. At a certain point of low air pressure the flapper will not close, so the last volumes of air can escape without the stick inserted.
I always try to remember to remove the stick, but sometimes forget. I was worried that leaving it in would compromise the flapper, but it has not so far...but still I try to remove the stick after I've squeezed the last bits of air out, just in case. I have not used the kites in sub-freezing temps yet so I don't know if the flap will get too stiff if the stick is left in, but better to just remove it I imagine.
As for having 2 valves...meh...I'm perfectly happy with the single valve...the more valves the more complication IMO..."LESS is MORE"
Hope you love the 15 as much as I do!