Sil 15% is a wonderful product, but if your doing copper to copper you could use dyna-flow that has no silver in it and with proper brazing skills it should last the life of the equipment.
The purpose of silver is in my opinion three fold,
1 The more silver the easier it is for me to braze and the joint looks nicer. Anything over 45% silver looks like jewlery.
2. The greater the amount of silver the stronger the joint
3. Silver helps with areas of vibration, such as discharge lines etc.
All this being said, proper brazing rules being applied, suction and liquid condensing connections could easily be soldiered with zero silver based brazing rod. In our town the local welding supply house gives samples to customers, they will even take you to the back room and let you try it. I would NOT recommend using a fluxed rod or that Black Max rubbish, I don't use any flux except when going from copper to steel and then I use 45% silver rod.
Information is available here; http://www.harrisproductsgroup.com/e...s/Brazing.aspx