The revs will drop down as you brake hard. It has something to do with the brake reservoir using air which the engine provides to move the diaphragm in the master cylinder...or something similar.. . That's why as soon as you have stopped, the revs rise again as it doesn't need the air as much to actuate the brakes. Sure someone else will define it better for ya, but as far as I know, it's supposed to work like this. Don't worry.