The pressure from the running pump is compressing some air somewhere in the system. Turn the pump off and the water runs back through it and up the cold feed into the F&E tank (or, alternatively, up the open vent and into the tank that way.
A big backflow means a significant amount of air - which is probably the cause of your other problems which seem to be airlock-related.
A big backflow means a significant amount of air - which is probably the cause of your other problems which seem to be airlock-related.