My North pump valve seems so have stopped locking off the air flow when I twist to release the pump hose.

Last few times I have screwed the cap on while it's still leaking air and it's held through the session

Is this normal? Anything I can do to fix it? I've tried blowing sand out.

sometimes you just have to change them out.

Have a read through this fixit guide (not my work):

ikitesurf.blogspot.com/2014/11/how-to-fix-new-north-valve-from-2013.html

The airport v1 valves seemed to more likely to leak than the later ones. North-tone claim that the valve isn't supposed to be the main seal and that the screw ring should be sufficient but........you'll rarely get a good enough seal on the screw top so its better to fix the leaky flap.

Or replace the whole assembly. Its not too tricky to stick a new one on. I think you can only get the airport V2 valves now anyway so its not a bad shout if you are looking to hang onto the kite for a while longer