2000 Cavalier LS, 2200 engine, 75k miles.

Today the car seemed to develop what sounds like a vacuum leak. I can hear it inside the car, coming from behind the dashboard and it sounds like it's in the area of the brake booster/brake pedal piston.
When I step on the brakes the rpms go way down and the car stalled once, so I'm pretty sure I'm dealing with a big vacuum leak, but any hints as to where I might look? I mean, are there any hoses coming inside the car that would have cracked or come loose or is this maybe the booster itself?

Any help or experience would be much appreciated. I'm about to go crawl under the dash to see what I can see...and hear.

I verified that it's definitely a bad power booster.
I've already got the new/remanufactured one and am trying to get the old booster out.
It appears to be glued to the firewall!!
After nearly 2 hours of prying I got 3 sides loose, but the upper left corner is just stuck!
I've sprayed a ton of WD40 back there to try and free it up. I guess it's rusted?
Son of a B$%$#@!@

Take some pics when you get it out and back in. I'm curious why it's stuck on there. I might need to replace a brake booster in the future.

The upper left stud was rusted to the firewall. I finally got it out by spraying back there with WD40 and using a pry bar stuck into the check valve hole and pulling down on it with a lot of force.
The booster itself was totally rusted through on the bottom, amusing a massive vacuum leak.
The firewall is fine.