Balls
Steve,
There are some small ball bearings, held in place by weak springs, that form the valves. You may have a piece of crud under one of the balls? If you have to open it up then you will almost certainly need need to replace any o rings. If you do dismantle it, the springs can shoot the balls a surprising distance, and the balls are small and difficult to find (don't ask me how I know).
Pete
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