XC90 Braking system
 

Just recently my brakes keep making a hissing sound and the sound only goes when I depress the brake pedal. Does anlone have any idea what the problem may be and how to remedy it?

Sounds like the ever-popular leaking brake booster.
Dig that hiss of vacuum.
You're in need of a new brake booster.
Ed

Second that diagnosis, unless you are not in the USA, you just need a new vacuum switch. You can't just buy the switch in the us, its illegal :/ look it up,i didn't believe it too. There are a few options.

a: dealer $$$$$
b: indy $$$$
c: diy $$

parts?
rebuilt will be in the $185 - 300 range
used from erievovo.com $275 with lifetime warranty
new $425 - 550

labor?
from $0 diy to $300 indy to 500+ from dealer.

i have heard dealer quotes of $900-1200

I choose the diy and rebuilt unit, if i had to do it again. I'd get the used lifetime warantyu from erievovo