Air leak noise from brake pedal

[clk320-]

Hi there hoping someone can help with this!
Occasionally when driving a very loud air leak noise appears from the brake pedal, and this noise disappears once I press the pedal and activate the brakes. Anyone know Why would this happen? And whether it is expensive to repair.
Then takes feel absolutely fine and the noise is only present occasionally a and disappears when I press the brake down.

Thanks very much

 

[The Dude]

...if brakes work well,strong retardation,firm pedal feel etc then this noise is normal, my Honda makes the same noise...

 

[rorf]

Possibility that the brake booster is leaking - it operates on a vacuum from the engines inlet manifold. How is the cars performance and does it idle well.

It should only hiss when you release the brakes.