Sealed cylinder losing air header

Hi all, we have a Tribune TXN180PT sealed hot water cylinder. It had an internal air bubble expansion accumulator, nothing external.

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Every few months I need to recharge the bubble by shutting the water off to the tank and holding the pressure relief valve open whilst opening a downstairs tap. It had always done this from new.

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I'm pretty sure I shouldn't have to recharge it so often, and there is no obvious way the air is escaping. The vessel holds pressure, so I'm happy there isn't a hole in it

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Does anyone have any ideas how the air is getting out, or is this normal?

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Thanks