Not all skillups are created equal. Most (but not all) skillups follow some derivative of the formula used for tradeskills. And this formula for some reason creates a "hell level" from 175 to 200. But I'll check this out and see if brass is moving slower than the other instrument skills at that gap.
Meanwhile, more info on the topic.
When the live devs tried to fix the "skillpoint hell levels", they broke all skillups and made it pretty much impossible to skill up with anything following that base formula, and they listed most of them. There is a thread where Ngreth discusses the difficulty gap, what caused it, how they broke it, and how they were fixing it. I can't remember where I saw it, but going by my history, it's going to be on one of these two links.
http://web.archive.org/web/200010181...qtraderscorner
http://mboards.eqtraders.com/eq/forum.php