This is for my gf's old school beater minivan. Braking in that thing is damn scary as it takes forever to slow down. Everytime I push the brake pedal I hear a short buzzing sound coming from the brake MC area.
I haven't noticed any leaks... could the MC be going?

 

How about checking the vacuum line to the brake booster... Maybe leaking?

 

i'm assuming the vacuum line is one of the hard lines going directly into the MC?

 

No - the vacuum booster is the big round black thing between the MC & the firewall. It should have a rubber hose (1/2"??) to supply vacuum from the intake manifold. Look for it leaking.

Let the engine idle, open the hood & listen while your GF presses the brake. Narrow down the location of the noise. If its inside the vacuum booster you need a new one. Usually you can't dismantle it.