Brake Pedal - Hissing Sound

[mercman1969]

I am getting a hissing sound (like air escaping) more and more frequantly
It generally happens after pushing the brake pedal and is almost as if the pedal is taking its time to return

The car is a W163 '52' plate

I know this is not normal as it doesnt happen all the time

Anybody have any idea of what it could be

 

[Fizzer]

Might be worth checking the brake servo. I had a car a few years back that sounded as if there was a snake under the bonnet every time I went near the brake pedal. It turned out to be a non return valve that was plugged in to the front of the brake servo. It will have a pipe connected between it and the inlet manifold. Could also be the servo diaphragm.

 

[SAFC]

I think Fizzer might have this right. However, there are lots of vacuum leads about behind the dash. If you've got a dodgy vaccuum-operated switch (e.g. headlamp positioner) and movement is just causing disturbance of the pipes (unlikely, now I think about it) then that might be the cause.
You're going to have to crawl about and hang upside down under the dash, I'm afraid.
If it's any help, my thumbwheel headlamp positioner went dodgy and hissed too. But this sounds more like servo-related probs. Best get on to it quickly, methinks.

 

[mercman1969]

Its definaly related to the brake
if it hisses, then applying light pressure to the brake pedal stops it

 

[andymerc]

my money would be on brake servo leak

 

[mercman1969]

Ok

So going for Brake Servo Leak....how Serious?

where is best place to obtain replacement? recommend a breakers?

 

[Bellow]

Check the hoses and fittings before you go much further. Could be a very simple thing like a split hose.