I've noticed that when I'm not able to ride for a few days, the front disc brake seems to get a light layer of oxidization on it and it rubs whenever I'm pushing the bike out of the garage. After a ride and use of the brake a few times, this rubbing goes away, until the next time. Is this normal? Is there something I can or should be doing to maintain the disc so this doesn't occur? Thanks for any help you can give.