My 2008 Ford Focus dies when I brake hard.

I'm not sure on what the fix is, but it is more than likely the brake power assist booster robing all the vacuum, air from the engine. There may be a problem with this, but I would have someone look at it to see if it is normal or not. I can create the same thing on my '95 Chevy and everything is new,

Carb cleaner needed, remove the Air inlet solenoid and clean off the carbon until the inner piston moves freely...typical maintenance.