i have an 03 blazer xtreme and ive had this hissing type noise from the top/center area of the dash when its hot. Ive noticed the only time it doesnt do it is under medium to heavy braking and engine has to be running. when the truck is cold its completly silent. so im curious if maybe the booster has an internal leak. Any advice would be appreciated.

 

Typically a hissing in the dash is the sign of a vacuum leak. I would check that out first. If the hissing is up near the top center, take a look at the vacuum actuator behind the glove box (there's two, actually). Very easy to access, just release the glove box door stop and let it swing all the way down. The actuator closest to center will be on your left.

I haven't done a lot of research on how the booster interacts with the HVAC vacuum system, but it could be that when the truck is warm and you hit the brakes, the booster is taxing the engine vacuum enough that there's less vacuum in the dash to suck in air and cause the hiss (although the vacuum reservoir canister should have enough reserve supply to prevent that...IDK). If you do want to check your booster, there's an old thread on the subject that might help:

https://blazerforum.com/forum/engine...booster-25898/