My wifes 2002 S60 AWD, w/77k has an intermittent hissing/vacuum leaking sound coming from behind the dash on the drivers side.

I drove it about a week ago, and when I was pulling it back into the garage, I heard what sounded like a vacuum leak behind the dash on the drivers side. I pressed down on the throttle just a bit and the pitch of the leak increased along with the RPM. I waited for the next day to look into further and I could not get the sound to return. The only sound I did identify was like air bleeding off when depressing the brake peddle. I don't drive the car enough to know if it's always made that sound, but it doesn't seem right to me in any case.

I've been driving the car to try and reproduce the leaking sound I had heard behind the dash and it hasn't come back yet.

I'm hoping to get feedback on further trouble shooting and diagnosing the problem.

Jeff

 

Its the brake booster, the diaphram is known to fail.. I had the same issue and replace the brake booster myself using the guide stickies on the S60 forum.

 

Indeed, the brake booster has a high failure rate in 2001 and 2002 MY.

Replace sooner than later as you may have a catastrophic brake failure when you most needed.

 

If it is the Break booster the Hiss comes from the Break pedal area. If this is indeed behind the "dash" and not at the Pedal i would look at the booster itself and make sure that either of the Vac hose are on tight.Most booster that go bad start with a small hiss at the Break pedal that when you apply a small amount of pressure to the breaks it goes away.

 

Had the similar problem and mine hissed when releasing brakes. Turned out I needed to replace the Brake booster. I was able to buy new one on line for very reasonable price of approx $188.00 USD.