Mystery rear flat that magically fixed itself

[Trebus]

[quote author=Buzz link=topic=24458.msg283191#msg283191 date=1534763660]
do you think it could have stayed slightly open when I first pumped the tyre then closed properly when I checked for a leak? 
Cheers...Buzz

sounds like a perfectly plausible idea to me. i can now see why you should always use a dust cap.
[/quote]

Had exactly the same issue. Took the valve core out and it was slightly corroded. Changed it and all well since.