Hello you need to change your master cylinder.
Brake linings need replacing? Vacuum leak?
Power brake booster is going bad. wont hold vacuum for power brakes to work.
Just know that if that is happening you are in serious trouble as it means that your brakes are leaking. In MLs the brakes use air pressure so if there is a hissing sound it means air is leaking thus your brakes could possibly fail. The best solution is to go to a registered Mercedes Benz garage so they can fix the problem. Alternatively if you dont have the money you could pretend that there is no noise and eventually you brakes will fail and you'll be in an accident i.e. GET YOU BRAKES FIXED AS SOON AS POSSIBLE
A ferret that make a hissing sound means they are annoyed or irritated
This sounds like the power brake vacuum booster going bad.
To make a humming or hissing sound, like an arrow or ball flying through the air; to fly or move swiftly with a sharp hissing or whistling sound., A hissing and humming sound.
Geese make a hissing sound.
Is there a grinding sound too? You may need new breaks. Take them to someone who knows about this stuff before they fail.
it makes hissing vocalizations
Snakes are reputed to make a hissing sound.
The power brakes on most vehicles operate on the engine vacuum. If there is a leak or problems with the power brakes it can make a hissing sound. But then, sometimes power brakes do that when they're working properly. It could be from either a hole in the diaphragm of the brake booster or normal operation. If the sound is new, and was not there before, and your brake pedal is going to the floor, then you should have the power booster replaced.
Could be a "hissing" sound