I've read quite a few topics on brakes making this hissing sound... like air is going out somewhere.

I had that started in my car first about a year ago... it was coming from right under the brake pedal.
There's some gray rubber boot that was somewhat misplaced I guess. I was able to move it back and
forward and I think I may have put it back in place, because I didn't have that issue for a full year.

Now it came up again. This time it looks like that rubber boot is slightly torn on the inside. THere's a little
piece of rubber that I can feel with my fingers hanging inside.

Here's the picture of what I'm looking at:
http://public.fotki.com/seabreath/al...o/img4568.html

I can't show you where that tear is, I will need to remove it ff the car first.
Anywasy here's my question... is there a place where I can get just that gray piece of rubber, without buying a whole
brake booster!? I called to local Volvo delaer, e-mailed them pics and they said it comes together with the booster at around $550.
If just putting that thing back was enough for a full year, I believe replacing that rubber should fix it too.

Besides that sound brakes seem to work as they should, they are really strong.

Can anyone recommend a place where to look for?
I'd even buy a used booster if I could get that rubber boot from it.

Meanwhile I'll try to remove and fix the one I have and see if that works.

 

I would try calling someone from the popular parts sites: FCP Groton or (my favorite) www.eEuroparts.com. I've called Dean at eEuroparts a few times and he's been able to help me find specific part numbers and I'd expect he could tell you if the rubber part is sold separately.
(800-467-9769) eEuroparts
(877-634-0063) FCP Groton

(Btw, the booster is only $472 at eEuroparts Part #30793677 - if you call give them the part # and that should help. I couldn't find it at FCP Groton)

If that doesn't work, I'd go the junk yard route.

 

The brake booster is what is making the noise. The boost doesn't seal anything.

 

I had the same problem, brake booster changed out, hissing stopped,
I have a RHD Volvo so the sub frame had to be dropped to remove the booster, cost me a F*****G fortune
They can't be repaired to my knowledge you must replace the whole unit..

Good luck
V70 2.4t