The brake booster is located directly in front of the brake pedal inside your engine compartment, right behind the brake master cylinder, the vacuum line is plainly visible coming out in front of the booster, with the engine running it may be possible to hear if it is indeed leaking. Also you can take the cover underneath the dash by the pedals and try to pinpoint where the sound comes from, I use a piece of hose, put one end to my ear and the other end towards the area where the sound is coming from, you get it close enough and it will lead you to the leak. Good luck