Definitely don't brush it, at least not the whole thing. My Arabian had a mane down a little past her shoulder, but because of lack of care it was completely tangled up. I just coated it in detangler, and with a small brush just started lightly brushing starting at the very tip. It was so bad that people had recommended that I just cut it all off and start over, but I didn't want to destroy her nice mane, and I'm glad I didn't. Hers was about 10x worse than your mares, and with just a little time, and lots of detangler/conditioner, I got it all untangled and looking nice. Good luck.