Quote Originally Posted by cudarunner View Post
Try taking Ryan's advice and use some shampoo on it.

Also try picking up a gallon of distilled water and use that for lathering and as a final rinse after initially rinsing the brush thoroughly.

Distilled water is cheap so give it a go! If it doesn't do the trick, you can always have it on hand for your car's battery.
If it goes away with distilled water then you know it is the hard minerals from your tap.