There was a great reply here once (with pics and all) re this problem but I cant find it

Probably so old it is now tucked away in a dusty archive box in Lauries attic.

Basically -
1) cut the end off a pump hose nozzle so that the end is a bit fatter than standard.
- cut where the new cut end only just fits at the very end of the valve stopper.
2) put the valve stopper; ie the male piece; into very hot water.
3) when its softened push the tapered nozzle into the valve stopper & allow it to cool.
May be a bit hard to reinsert into the valve - but only the first time.
Works a treat and you can do it as often as necessary.
HINTS :
1) If the cut nozzle is rough or sharp or a bit too big to insert into the valve end easily you can smooth & taper it in a pencil sharpener.

2) I keep my short mod'ed nozzle in my kit. Every now & then I do the above by just warming the stoppers in my mouth. Keeps them snuggly tight

all the time.
Never had to do the hot water thingy since I started with the preventative treatment.