Good Evening Everyone!

I have a 98' ML320 and recently it started having some brake issues. In addition, my BAS ESP light came on in the dash. The reservoir is full with DOT4 fluid and no leaks were observed on the brake lines. When the car is off and I repeatedly press the brake, the brake pedal attains pressure. As soon as the car is turned on, the pedal becomes very soft and can be depressed to the floor. Everytime I depress the brake you can hear the pedal hiss, I always attributed it to some air being in my lines. I have heard that such issues may be caused by either the master cylinder or the brake booster itself. If anyone has any tips or fixes that would be greatly appreciated!

I have also added a link to an unlisted youtube video showing my cars pedal hiss.

 

Welcome! Please post a photo of the ML. There is a model-specific section of the site where, if you post the question there, it will receive faster replies from knowledgeable owners.

 

Just added a picture of the vehicle! Thanks for the help!

 

Looks great! First vehicle in its class when introduced!

 

moved to W163 forum...

 

You have the dreaded brake hiss issue. This is a common problem on the w163s. Yours sounds pretty bad.
If you get under your DS footwell, look up where the brake pedal is connected to the BB on the firewall, you will see a rubber boot.
You have a vacuum leak at the junction, and there are several possible solutions.
1) remove and replace the BB,
2) remove and use a BB repair kit where the seal itself is replaced,
3) the repair can be done from the underside in the foot well, depending on the extent of the leak.

We resolved ours with method (3) - following this except we used a piece of bicycle inner tube for the fix - https://mbworld.org/forums/m-class-w...rt-help-3.html
The other alternative material suggested is layered plastic food wrap (to seal the vacuum leak).
Should this fail, we have a backup BB to replace it with. It stills on the shelf for that possible day.Repair
Repair kit seal info listed in that thread too.

 

Hmm. I may give it a shot! I feel like removing the brake booster may be a challenging job especially with the throttle and fuse box in the way. Not to mention the need to potentially have to disconnect my master cylinder which may lead to me needing to bleed the system. Do you have any more tips regarding the repair process? Also any tips on avoiding the potential breaking of fragile plastic parts? The car has well over 220k miles so some of these plastic parts have become very....sturdy so to say

 

Yes, plastic will be brittle especially under the hood.
If you do a search on for diy videos on youtube, or step-by-step procedures, etc, on how-to remove the brake booster, it is possible (have not attempted this myself....) to NOT disconnect the MC. The space is tight to remove the BB but doable. I would be more worried about the disconnecting of the plastic components vs the procedure. If it breaks, make sure you have spare parts on hand before proceeding.
I would take some time to review these to better gauge yourself and see if this works for you.

As for procedure (3), rake the DS as far back as you can, get on your back and crawl in (my old back survived the one-time procedure....) and study look at what space (limited) you have to work with on the underside.
Remove whatever covers etc that are in the way to give yourself space. Look and watch closely as you depress the brake pedal to see exactly where the hissing is originating from (most likely along the firewall where the end of the BB pokes through. You will need to pull back the boot cover to see and listen for where the hissing is coming from. I would wipe down that area all clean of grime and grease, if any first. I had some white grease spattered up that area when I tackled the issue, How that grease got there? Most likely a lazy tech getting excess lube up there for the brake pedal components (just guessing, who knows)
Once you have determined the possible hiss location, you can put back the rubber boot back in place or not. We chose to put it back in place before wrapping it.
This will vary from case to case, depending on each scenario of where the hiss is coming from and condition of the boot cover.

Preparation:
Get you zip ties ready to use and had our inner tube pre-cut to what we thought was the correct sizes (we cut a few different lengths and widths and had them handy since I was going to be on my back in the footwell. I didnt want to get back out each time. I'm lazy.


In our case we used bicycle inner tube, pre-cut to size and length to use to wrap the tube-end of this BB that was poking through. If I had to do this again, I would use some kitchen plastic wrap.
Once it's wrapped, zip tie tight against the firewall and again about 1/4 inch or so away
Once done, test to see and listen.....
You may require a few attempts to get it wrapped and sealed correctly.
We got it done on the first attempt - it's been trouble free since.

 

Hello MBForums!

Bit of a quick update! I tried to find a new booster near me but failed to do so. Went over to a local scrap yard and took one off another ML320. After much cussing and slightly bloody knuckles I managed to both remove and replace the brake booster itself. Seems to have been an internal failure of the booster itself rather than a simple air leak through the gasket. The ML is now back up and running! Probably going to give the fella a brake bleed in the next coming days but that should finalize any brake issues! Hopefully. Thank you all for your responses!

 

Glad it all worked out for you. Happy Driving.