Mine was like this it was the strut leaking but there were no bubbles externally or hiss to be heard. The car would prediocally top it up from the pressure in the reservoir for about 8 hours.
I replaced the airbag and aluminium housing, the airbag was in perfect condition, I think the leak was at the internal o-rings
I replaced the valve block first to no avail that is easy enough, just need to be scrupilously clean when fitting the connections.
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