I have a leak coming from the screw on the isolating valves on either side of the circulating pump. The pressure has gone to zero - I have tried repressurising the system using the filling loop, but more water comes out of the valves screws (the screw that opens and closes the valve). What could the problem be? Is it the valves or maybe the pump? The leaks stops when the boiler is running, but starts again when the boiler is switches off.