When you're braizing copper to brass/bronze, iron, or stainless steel. The prefered method is to go with a BAg (braizing silver) alloy 45% and to use a braizing flux/paste. I would wrap the valve in a wet rag so as to avoid damaging the internal parts and watch your heat it is easy to overheat the metals and burn the flux.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.