You shouldn't brush your horses tail at all. You should apply a detangler and then pick apart the hairs a few at a time. It doesn't take as long as you'd think. Brushing causes breakage.. braiding too tight also causes breakage.
I prefer shampoo, condition let dry.. braid and loop and wrap with vet wrap and then wrap the closing with duct tape to make sure it stays. I've had mine in for almost 2 months. I do this for the cold months since I can't wash them as much. I also rub MTG about once a week on their tail bones.. Their tails have improved soooooo much.
In the summer I do the same thing but just cover with a tube sock.