On an open system there often isn't enough pressure to force the valve shut again. All it takes is a small bit of dirt and it'll start weeping a bit (usually doesn't pour out).
You don't say Sherlock. Voice of experience I take itI think the Altecnic Robocal AAV is good. Always keep the black screw in place unless you think there's air to purge (ie don't lose it).
I'm going to fit it to an isolation valve as suggested and just use it now and again to vent the system
This should stop any leak problem