I have a 2001 Volvo V70XC. It has just 0ver 94k miles on the odometer. It has developed an infuriating hissing sound associated with the brake system especially in warmer temperatures. My first guess was the brake booster but leak tests reveal no leaks. Then I suspected a vacuum hose leak, but I cannot locate one. An ASE certified mechanic also could find nothing wrong. If anyone has any ideas what could be causing this hissing sound emanating from the brake system I would appreciate hearing from you. As I mentioned the hissing gets worse the warmer the outside temperatures are. Thanks

 

It's the brake booster.

 

If it is the brake booster why do leak tests reveal nothing? Also the hissing is absent in cooler weather and becomes more noticeable when the temps outside go over 80 degrees. If it is the booster should the hissing not be present at cooler temps as well? I am hesitant in replacing the booster only to find out it is something else that is the cause of my problem. The booster is a fairly pricey item and labor intensive to install.

 

How are they "leak testing" the booster? You say there is a hissing associated with the brake system... how are you coming to the conclusion it is associated with the brake system?

 

In retrospect the hissing sound may not be the brake booster after all. My Volvo Tech has done a Vac boost operational test, a booster reserve check, a check valve test and a hose leak lest from the PCV valve to the check valve test. All are negative. As I said the hissing is pronounced at higher outside temps and is really evident when it is really hot outside. Above 90 degrees. All other vacuum hoses check out as well. The hissing is momentarily interrupted when the brake is applied and immediately resumes when the brake is released. The brake pedal does not become hard. In the absence of any vacuum leaks in the hoses and the fact the brake pedal function appears to be normal I am at a loss to explain what is causing the hissing. At this point it is more of an annoyance than a problem. Perhaps this hissing will be diagnosed when something fails. But that is something I do not want to happen as it may be a catastrophic brake failure at high speeds.

 

It's the booster.