Replacing air-(release?)valve on gas boiler?

I have a newish boiler (gas) which has at the top a brass gizmo that (is supposed to?) release air trapped within the system. I think it's duff...but just below it is a little tap which, it I open it and have a bucket handy spurts out hot water + air. The boiler 'gurgles' along, so I'm guessing I have air in the circulating system. I've bought one of the litle grass canisters, which has a plastic screw on top piece and inder that it's like a bycycle tyre inflator on the tube.
I'm in France and have tried to make sense of the plumber but it's become 'complicated'! He's a long way away and simply can't get here. I assume I have to drain the system before unscrewing the old device and popping on the new one? It's called a 'purgeur' which makes sense since it purges the system of air!

But any help/advise would be appreciated.

Thanks.