.

Once again, we don't have enough context to understand exactly what is going on here; but my informed hunch would be that this is about in some way preventing a suction pump from becoming un-primed — so it remains 'full of liquid'; I don't see any relevance to the word 'charge' here, unless the wider context indicates that?
Do note, however, that my suggestion is only to give the meaning — the register is not appropriate for a patent translation, but the proper expression can only be found once the wider context is known.

This is also about a priming tank I suppose?
This was my first idea and only then did I read Tony M's comment.
So I've posted it and hope that Tony M is okay with this?