I have two floors jacks one is a cheap Costco made in China & it works fine.

The 2nd is an American made Blackhawk jack. It jacks but will not hold any weight. It immediately starts going limp as soon as you stop pumping it. Doesn't appear to be leaking fluid. Maybe it has air in it or something. Is there a way to bleed a floor jack?

Your seals my be shot. How old is it?
There are tool repair shops that can rebuild them with new seals. Rebuilding a Blackhawk is worth it, I was told it is not worth rebuilding a Costco China model.

Paul