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From what you wrote I am guessing you are not building electronics with them..
So - Just don't let the air inside..
The metal plating inside (it is not silver) has a specific purpose: once the air is pumped out and the valve is closed, this metallic layer is deposited on the top of the valve.. and it additionally removes the remnants of the air (oxigen) and thus lowers the pressure a bit more.
When you let the air in, this layer becomes white.. it is turned into oxyde.
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