Thanks Alliance. Nice to be vindicated instead of flamed for a change

@ NickT
I assume you mean the female stem in the boiling water. I wouldnt know cos Ive never tried ? But I will now.

Best fix I know of to avoid a stick on replacement is only for those kites that dont have a solid male cap.
Put the cap in boiling water. Then insert your pump nozzle inside the cap to stretch it wider.
(you may have to trim your pump nozzle end to get the right dia.)
Wait for it to cool or put it in cold water.
Cap will then hold in the stretched valve stem.
Works OK - but not so good as a long term result.
If you have a solid core cap you can try swapping it for a hollow centre one.
Most shops would have them laying around the tool shed as freeby handout.