Not a big problem but irritating. My tire brushes build up with black greasy substance after so many vehicles. Got a solution? I figure the tire dressing from regular customers or diluted cleaner are to blame. I'm looking to keep costs low in that area. Currently using AM RedHot (3 or 4:1 which works out to about $1.50-$2 a gallon) and Dealer Choice Water based dressing from AM distributor ($10 a gallon in pails). I'm fixing to restock on my general cleaner so if anyone is using something else from AM let me know.