Hissing noise when Applying Brakes

[HTX86chevy]

Ok, So I did some research on this site and the net and all signs point to Brake booster needing replacement when hissing noise results from applying the brake.

I tracked the noise down to this little switch under the dash. the noise only occurs when the brakes are being pressed, such as at a stop sign or stop light, but does not occur when slowing down without coming to a complete stop. this switch seems to have a vacuum hose connected to it. anyone out there know what this switch does?? have any idea why its hissing?

brakes still work perfectly, but the engine idles roughly once the hissing starts. gasping for air?
the 1st link below is to the video. the 2nd link is to a pic of this switch.

http://youtu.be/-zBhwsxvcns



https://drive.google.com/file/d/0B5n...ew?usp=sharing