I've got a boiler and expansion vessel upstairs, and a few days ago, notices the upstairs shower had crap pressure. The pressure gauge on the expansion vessel shows next to zero, so it seems I need to repressurise it. There's a filling loop attached (as seen in the photo), but only one black valve, and turning it does nothing. I've detached the filling loop, and nothing comes out when I turn the valve. I don't see a second black valve anywhere. Any ideas?