I know whenever I press the pedal there is a noise
because of the brake booster. But from yesterday as the weather gets cooler here in Southern CA I hear a similar noise “hissing noise” when I release the pedal as well. When the weather gets warmed up a little the noise is still here but gets much quieter. Is this something to bring it in? or is it normal? For the year I owned it
I never noticed the noise when I release the pedal.

Thanks

I am in the exact situation right now. Still trying to determine if I should bring it in.

lol if you do let me know.
I don’t have time until early next year to go;;

I've heard that noise from time to time. It doesn't seem to be a problem. I've been driving my C400 for more than 3 years and the brakes work great regardless of the intermittent "hissing." I never asked for it to be checked.

My C300 doesn't make any noise and is very smooth......my chevy truck brake pedal on the other hand squeaks, groans and hisses so much I have a hard time driving that beast.

If it is a small "squish" sound - then yes - that is normal - it is not your brake booster - it is some of the elements in the leverage system

If you have a question about it - bring it up as a item on your next service

I don't know if i'm just noticing it more now since I saw this thread but I hear it too and i'm fairly certain it's normal but probably exacerbated by the fact that it's 10 degrees here and everything has to work harder.

I do love a good bit of guesswork.

the truth however is that it’s a bypass valve to allow the brake hold feature to work.

my c400 makes this hissing noise when pressing and releasing the brake pedal when its cold out (-5 degrees celsius and colder)and its contant..on warm days I don't hear it..i know this isn't normal becauz currently I have a 2018 c300 as a loaner while mine is in the shop or other things and it doesn't make any noise when you press the brake pedal and its damn cold out!

what was the part on yours that was fixed for this issue? it always seems like I have to tell the MB tech what part it is that has to be changed lol..

someone mentioned brake booster on another thread but not sure..?

I haven't updated this thread for a while.
It turned out to be the leaking brake booster.
MB of Thousand Oaks in L.A area replaced it under warranty.
If you guys have that issue check with your dealer and get it replaced.

mine they said is little louder hissing than "normal" so they mentioned will replace brake booster (part is now ordered) will keep updated.

Tell them to replace it.
I'm glad that MBUSA purchased my lemon C300 back.
My new Audi A5 Sportback is awesome and most importantly trouble free.
I was hesitant with VW products but it seems to be rock solid.

160 days in service.

MB refused to buy it back voluntarily.

I hired a famous lemon law lawyer.

It took almost year and half but I was able to get every penny back from the purchase.

How much did the lawyer cost?

Lawyer was paid by MBUSA.
If you win they get paid. if you lose they don't.
I saw the settlement paper and it stated something like $50K in legal fees.

CPO is a little different than new. But it doesn't hurt to contact a local lemon law layer.

160 days total. Longest was 35 days and I got to drive a loaner GLE 350 with 50 mines in it.

That made me not to get the GLE as a replacement to my old ML.

A lot of things.
It popped up as soon as things got fixed.
My worst issue was key not detected issue.
MB replaced the key 3 times and didn't fix the issue.
That 35 day was 2 keys ordered and a keyless go receiver from Germany.
Brake booster that I originally wrote in this thread was 2 weeks.
My Steering column module broke down the first month of ownership which took 25 days.
It was a disaster.
Ironically my Hyundai Genesis that I purchased in the similar time period only needed a recall for the MDPS (electric steering system).
Audi A5 sport back so far has zero issues.
Lexus RX not even a single noise... very impressed.
Even my dad's 2016 S550 had no issues so...
W205 seems to be a bad model for MB