FWIW, if CB do a 30.9, there are probably a dozen or more broken ones in boxes within 50km of where you are, just stick a wanted for spares or repair ad up on STW or on here and you'll get a good few responses!
Indeed, there's a 30.9 Joplin in my loft that just will not hold air for any more than 10mins of riding, thought it was valve, replaced, no luck, so must be a seal at the top, I lost interest in fixing it about 8months ago.
Did you rebuild yours with a seal kit? We could come to some arrangement over swapsies on the 30.9 lower (upper assembly should be the same) if you felt like rebuilding mine for me too ;-)