I was getting this.
I decided it was the rubber seal around the inlet valve, I wonder if these can be found separately?
But, what I have found seems to work for me is.
Put the tank in. give it a firm push down to get the suction sound.
Then, give it small taps with my hand around the edge, you may see some small air bubble float to the surface (I do), this seems to have stopped any leaking for now.