2003 Other Toyota Models Toyota Sequoia

I have a constant sound like air leaking - kind of like a hissing sound. It stops when I brake, but otherwise it occurs all the time. I took it to the dealership and they told me it was my brake booster and that there was no safety issue, that I could drive it like that for years w/no further problem. I'm not comfortable with that answer as it is loud, aggravating, and I'm afraid I'll get stranded. I use my car for business and don't need to be on the side of the road with clients. Any idea if the dealership is telling me the truth, and am I in danger?

Thanks so much!