I took my car out of winter storage to find chunks of rust coming off behind the disc plate of the brake. The 'drum" area. Is this normal? I am not sure if i used the write wording but behind the discbreak plate that its fastened on to there is another disc and its very rusty with rust coming off and sometimes in chunks, is this normal erosion or could this be a serious problem? and the break seem to be still working fine. thank you