1998 SL 500 Hissing sound
 

The hissing sound stops when brakes are applied. Then begins again when you remove foot from the brake peddle. What is causing this and is it an easy fix?

hissing
 

Is probably the vacuum to the brake booster what you are hearing, it could be as simple as a leaking vacuum line or as expensive as a cracked diaphragm inside the booster that will require replacement, how is the idle, any check engine on?

The idle is fine. No check engine lite except for the ocasional one that pops up from the gas cap. You can hear the sound coming from around the brake peddle and as soon as brake is applied it stops.

Idle should be around 600 rpm in park. Is that what you see? I really quick way to check for vacuum leaks is to use a can of starting fluid (ether). Get some at any auto parts store. With the enging idling, spray a little on your vacuum lines, one at a time. If the engine idle suddenly jumps up momentarily, that line is leaking. Obviously try not to let any go directly into your intake, which will give you a false indication.

Can you direct me to a diagram of the booster to determin where the vacum hoses are located?

I know this sounds a little strange, but where's the sound emanating from and how load is it? Inside or under the bonnet (hood)?

The headlight position switch is vacuum controlled and mine hisses in position 0 (default) and stops occasionally - normally when I turn it to pos 3!
It's a very light sound which is just annoying rather than shouting at me...

Just thought I'd mention.

Lea

hissing
 

The brake booster is located directly in front of the brake pedal inside your engine compartment, right behind the brake master cylinder, the vacuum line is plainly visible coming out in front of the booster, with the engine running it may be possible to hear if it is indeed leaking. Also you can take the cover underneath the dash by the pedals and try to pinpoint where the sound comes from, I use a piece of hose, put one end to my ear and the other end towards the area where the sound is coming from, you get it close enough and it will lead you to the leak. Good luck

Thanks. After a closer inspection yesterday it sounds as if the hissing sound is coming from the booster. It's not easy to hear when the car is running. The vacum tube is is visable thanks for telling me where to look for it. I will lower the facing plate above the brake peddle to see if I can zero in on the sound. On the other comment about the light. The sound clearly stops when the brake peddle is applied and starts again once it's released. Thanks for the thought there. Will update on my progress. Lastly if it is the booster is this somthing that an novice can replace or should it go to the shop?