I have a 2001 Mazda Tribute. I have noticed a hissing when I apply my brakes in recent days. Sounds like a vacuum issue- like pressure is being equalized. It only happens when I depress the brake pedal however. A little looking around makes me think it might be related to my brake booster? What is this?, is it dangerous to drive with this going on? How hard is it to replace? I am amazed at how many things can go wrong with this vehicle. I bought it because my gmc was repair prone- turns out it was a dream compared to the tribute. And it's not like I bought anybody else's problems- I bought it new!

 

It's the brake booster, my 02 Trib made this noise and replacing the booster fixed it. It's not too difficult, just four nuts on the inside of the firewall hold the booster in place; disconnect the pedal link and master cylinder and pull it out. You have to disconnect the brake lines and completely remove the master cylinder so be prepared to cap off the brake lines and replace some brake fluid that will leak out. Took about 1 hour going slow and paying attention to how things need to go back together.
Anyway good luck with it.