you've probably got this solved by now, but I'd higly recommend against making an automated expression for breathing unless you want your character to look mechanical.

Breathing is a big part of bringing something to life.. you can use it very smartly to help define what a character is thinking.. how they're reacting to something.. etc. Imagine the difference between the following:
1) The character's face is passive but they're taking long deep breaths
2) The character's face is passive but they're holding their breath
3) The character's face is passive but they're taking short shallow breaths

all three represent something completely different.. the first, the character is at rest.. relaxed. The second, they're scared.. possibly, or maybe a bit startles. The third, they're definitely scared or excited about something.

Just a quick tip.