This sounds like a seal that is leaking on the end of the servo - if you remove the under dash trim you may be able to listen or feel for the leak to confirm around the rear of the servo.
The brake servo is mounted to the bulkhead with a vacuum connection & the rear of the servo coming into the inside of the vehicle where the brake pedal attaches to. If the seal fails then you will get a hissing noise as the air is being drawn in past the seal inside the car.
If the servo is at fault then it should be replaced as a single unit. Sadly they are not overly cheap as they contain electronics for the BAS (brake assist) system.
Best regards, Paul
Hi, by the time I got your answer I'd already been to a local garage (couldn't go last night as it was past closing time and I needed to know quickly whether or not it was safe to drive the vehicle). However the mechanic I saw has made the same suggestion and asked us to have it confirmed by the Mercedes dealer.
No problem, good luck with the repairs! Let me know if you need anything else.
Thanks - I asked on 'Just Answer' because I needed a quick response and I didn't get it til the next day. Am I justified in not paying or do you expect payment anyway? Just want to do what's right here as I may want to use the service again.......
You only need to accept if you are happy with the answer, there is no pressure to accept & if you feel the answer did not help you then thats fine not to!
Let me know if you ever need anything else - the speed of the answer will depend on how many experts are online at the time of your question & the knowledge of the experts for your question (ie Mercedes specific). I am on pretty much most days but do work also, so if you do need anything else I will always try to answer as soon as I can.