Houndmidas

Hello all. So I just acquired a 2003 ML320 (AMG/Sport Style package) at an 88000 mileage. I've had it for 6 months and everything's been great so far!

About 2 weeks ago, I started noticing a hissing sound emanating from the brake pedal at various intervals of decelerating. Usually at points where I'd left my foot on the pedal for an extended period of time, and then released it. For example in heavy traffic jam. I took it to my local dealership and inquired about this irregularity; most of the mechanics were clueless, and one offered up an explanation about the brakes doing this periodically for self-maintenance. Absolute bull**** of course. . They all told me not to worry and that there was nothing wrong.

Well, long story short, after doing massive research on these forums and Google in general; I've found out that I most likely have a leaking diaphram on my brake booster. I even found video that reproduced the exact sound I hear. I have a very busy schedule, but I'll be taking it to a more qualified garage in due course.

My question is, what are the effects of this? It doesn't affect the car's operation, and it operates with no hitches. Is it dangerous for me to drive the car with this condition? The posts I find only offer up in way of negative implication, the annoying/irritating hissing sound.

Any tips/advice here?

Maj. Dundee

As you already know, you are leaking vacuum at the brake booster which can become dangerous during panic stops.

Either replace the booster with a used one or remove the booster and replace the seal.

khomer2

Several options to consider.
but firstly, very surprised the st_dealership mech techs you seeked advice from on this hiss were clueless. (Run away from them fast and far.... and dont go back imho)
The brake hiss is a known issue on the W163s.

You have the option of DIY replacement of brake booster or URO seal part replacement (I've done this on a spare brake booster I have just in case I need to swap it out), or patch it from the underside in the driver's foot-well (as shown in the visual of your first post).
Depending on where the specific leak (hiss) is coming from, you can patch it from the underside (as I have also done reading the many DIY on the forums) and seal the hissing leak. Posted my solution (using bicycle inner tube rubber and ziptie) without removing the brake booster (posted on the other MB forum) and it's held up successfully without any issues of hiss or affecting the braking. Just had the SUV in for a complete medical checkup with our veteran MB mech before the winter season hits and all is good.

As Maj,. Dundee has said, it could become an issue as he describes. Personally, did not have any issues with this (other than the annoying hiss interfering with our audio tunes), our colleagues with same issues in the clubs also found it annoying in their MLs and most never resolved the hissing, however, no braking issues that we heard about locally. That being said, the hiss is still a real annoyance so I would still repair it whether DIY or at the shop.

Houndmidas

Thank you both for your prompt responses, very helpful and appreciated!!!!!!!!