it would be to do with the brake booster as it has a vaccum feed from the engine. the higher the vaccum, the more power it has.
as for pumping the brakes, pretty sure that's normal as you are just pumping up the pressure in the system. and i have a feeling that the brakes in my commodore do a similar thing as yours do when pushing the clutch in, etc, when the vaccum changes (such as turning on the aircon, etc, since it's an auto)