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I have offered this solution before: buy a gallon of distilled water, put the distilled water in your soaking cup to what level is normal for you,
put the cup in the microwave on high for a minuet,...

If you live in a place that has hard water you might try this. Buy a gallon of distilled water. Put
the amount of distilled water in your brush soaking cup; put the cup in the microwave, on high...