Volvo: brake booster hissing sound, fuel mixture, brake booster

Question
Is it safe to drive with the hissing sound? Is this a repair that needs to be done right away? The brakes work fine but the  hissing sound is annoying.

Thanks

Answer
Jan,
This means there is a leak in the brake booster, and that should be fixed asap. There is no way to know when the leak will get worse making the brakes very diffcult to apply. Could happen at the worst possible time, during a panic stop or school cross walk. Also the leak is causing air to be sucked in by the engine and that is even in a small way affecting the fuel mixture. If your car is newer than say 89 or 90 the check engine light could come on with a code for fuel trim or air mass meter.