Yes, you'd certainly shoot it empty of air before attempting to unscrew the Foster valve. And then go very carefully as you unscrew it, to satisfy yourself that there's no remaining pressure behind it.

From memory - Yes Brocock use a similar valve to the BF item as linked above.

The BF valve is strippable / serviceable, as I would imagine the Brocock valve is - so I doubt you need to order a new valve.

If the slow leak is from the center of the valve - that's commonly caused by a tiny bit of grit ingress, or a dry / perished seal in the valve.

If you're removing the valve, you might want to try blowing it through with a drop of Pellgunoil or silicon oil.

You'll see what I mean if you Google "foster valve service" images.