I tend not to brush the tail too often, as I agree that the brush will break hairs and make it thinner. I have untangled many a tail by using my fingers, from the top one hair at a time. You end up with black horrible greasy fingers and the horses tail looks lovely.
I would then try to spray in a detangler or baby oil or even some human hair conditioner. That would keep it tangle free for a good while longer.
The one hair at a time thing does take a lot of time but I've usually done it when heading for a show as it does make the tail look fabulous. Its a nice thing to do on a hot summers day too, when you're just hanging with your horse.
In the winter when theres mud clinging up the tail I would just brush the bottom out to get rid of the worst tangles, I also tend to trim the tail shorter in the winter to get it out of the mud.