May be a novice question but when you say brush are you talking about a brush in PS not in AE. If so my guess is did you just erase the file in PS in steps save the file out each step then animate in AE.
Also if you have a link to your final preoject that would be super sweet. Ive been looking for some referral videos but not much luck in that department. i understand if not.
thanks.
aaron