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This problem happened to me this morning, pushing the button did nothing to help, even after borrowing some pliers from a lawnmower shop and tightening the valve core. And removing the hose from the pump itself. Tried again with the pliers, fucked up the threads and bent the stem so couldn't even thread the pump hose on or tighten the knurled nut. Luckily they had a compressed air line so I jammed some silicone tubing over the whole valve and managed that way. (I should have explained what I needed the pliers for when I first asked, instead of looking like a twat pumping madly but getting nowhere (no air?) with my stupid little pump).
So I have just had a look at my tubes and pump - the pump hose head screws directly onto the valve core threads, which on these tubes (Continental Race 28) is the SAME thread the cap screws on to. There is a flat side on the threads to help screw the core into the stem. You can screw on the cap and pull the core out of the valve body if loose enough. Neither the cap nor the pump hose engages on the valve body outer threads at all and there is no other thread. If the core is really tight you can get away with it but it is not just a case of the seal and pressure with these tubes.
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Egghead
The hose doesn't bolt on to the valve core thread. It bolts on to the cap thread, below it.
It's the pressure in the hose and friction between the hose seal and the core head that causes the core to unwind with the hose. @Ramsaye Pressing the button dumps the pressure from the hose, and hey presto, your valve cores don't unwind anymore.
Well, this was my experience after suffering from the same problem as you, chatting to mr. Lezyne about it, then having the problem go away :)