I am having a strange problem with my 2000 S80 T6. While driving I am experiencing a hissing noise coming from under the dash/foot well on the driver's side. At first I thought that is was a speaker picking up road noise but then I realized that no it was not a speaker, it was coming from under the dash. The strange thing is that if I even barely tap the brake pedal when I hear the hissing it stops. When I release the pedal it sometimes comes back right away, sometimes not. In addition to that I also get a high pitch whine that acts the same way as the hissing, but it is tied into the gas pedal. Both of these noises occur about 60% of the time everytime that I drive. Does anyone have any idea what is going on?
Thank you in advance,
-J.P.

Had to have my booster replaced on my 1999 S80. Same thing. Started making a hissing sound when I pressed the brakes. I didn't have a total brake failure, but did start having trouble if I had to hit the brakes suddenly. So take it ASAP to have it repaired.

Is this a common problem on Volvo S80's...reason i is that I have a few friends in the UK who have had to change this servo around the 65K mark....and what can one expect to pay Volvo for replacing the unit? Thanks in advance:)

docvb is correct, I just had my booster replaced today...had the same problem

Well, both of you were right. I took it in and they say that my booster needs to be replaced. I was told however that over time it would just get harder to brake, not that they would fail. Is this true? As a result, I did not have it done right away, I figured that I would just wait for it to go. Any suggestions?
-J.P.

My Volvo has just had new top rubber mounts fitted and the top engine mount,afterwards the car developed the same hissing noise...the dealer can't find the problem....will have to check out what Frank suggests!Thanks for that Frank:)Fingers crossed!!

I suspect that the outer steering column "slip ring" has moved out from the booted section where it goes through the firewall. This happens everytime someone lowers the subframe to either repair the trans, replace the trans or replace the steering rack on S80s. Very often the tech forgets to check that slip ring and re-install it. It takes only seconds to re-install, it just slides back downwards under the rubber boot lip.
Another possibility is that someone checked or replaced the air filter and that they failed to properly re-install its upper cover towards the drivers side. It's a very poorly designed air filter box and many who replace or check the filter fail to get it re-installed correctly and if so, it leaks air and you can hear that air leak noise at full throttle.

Thank you very much Frank.
That makes a lot of sense, because it has been occuring for about a month now, and I just had the engine sub-frame lowered to install my new stiffer sway bars about 2 months ago. I hope that that is it. Thanks a lot again,
-J.P. Sollock

That's your power brake booster leaking. The large membrane is torn and you are hearing a vacuum leak.

Proceed directly to dealer. Your brakes may fail. Like mine did.

Docvb