I've not found a need for a brush. I usually mix up a couple gallons of PBW solution (using 3/4 oz per gallon, I think) and soak / vigorously shake it. Usually after less than an hour, the last krauseny bits have relented and it's crystal clear. I don't usually go past 3-4 weeks in primary, though, and many times I rack off after a week or two, so the krausen doesn't have all that much time to harden. In one case I had to soak/shake for a couple days before the last bits came clear, but the PBW was victorious in the end. If you're doing longer primaries, you may have more problems.
To avoid needing to completely fill the fermenter with throw-away cleaning solution, I usually only make a couple gallons. The only place that ever needs soaking is the top part where the krausen dries on, so I just pour in the solution, firmly plug the bottle shut, and flip it upside down over the sink or over a bucket in the garage. This saves both water and PBW (not such an issue if you're using Oxy Clean I suppose).
