Hello everyone, wanted to show this video where I replace the leaking plunger seal of the Brake Booster without removing the Booster from the car. The video was done on a Volvo, which uses the same ATE Booster as several Mercedes models from around 2000-2014. Hope it helps everyone with this issue

I'm posting this also on the W204 forum since both models had this booster

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Hopefully they've improved the seal in the updated brake booster. There's now a new PN for the entire assembly.

When mine started leaking (slight occasional hissing sound) I just jammed a pile of thick silicone grease into the seal. Now there's only a slight hiss when the pedal is being released, but I can live with that.

Packing with silicone grease can surely help limit the leak. Also good idea to put quality some silicone grease even if the seal has not started leaking - however I believe the main issue with leaking is less the lack of grease and more the fact that as you mention, the rubber in the seal gets too soft over the years, which obviously prevents a proper sealing given the quite important vacuum pressure it's submitted to.

Looks like the seal is really starting to go in my car. Anyone successfully replaced this seal without removing the brake booster from the engine bay? Also which specific replacement part was purchased/used?

Years back I bought a used booster and had a mechanic install it
I didn't want to attempt it because the WIS docs showed disconnected all the
brake lines from the ABS pump, but the mechanic did NOT do that.
He moved the SAM/Fuse box out the way without disconnecting anything, and got it in and out
in 2-3 hours total and charged me $600. Nice guy( not, he charged me for 6 hours and was done in 2-3).

I just did the job inside the car. Here's the DIY link.

Thanks for sharing. Are there any symptoms that triggered the seal replacement (other than the potentially annoying hissing)?



I’ve had issues with my transmission 722.6 which I’ve thrown everything at. Some suggest this vacuum leak could be the problem so wondering if the leak had any impact on transmission or the car more broadly?

Can you be more specific with regards to the transmission issues you're having?

On my 722.6 5-speed (same as yours), the car has always shifted rough if a) the engine isn't warmed up, and b) shifting into 2nd either upwards or downwards. It was most noticeable after I had the transmission fluid changed (full change, to the newest 236.14 fluid). I thought that replacing the booster seal would fix this problem, but it didn't. Having the constant hissing sound go away sure was nice though.

If you're talking about the car randomly dropping out of gear, the problem has to do with the leaking pilot bushing (bad o-rings) which soaks up the attached wiring harness. The newest bushing from the dealership fixes the problem, as it comes with updated orings.

Exact same problem as you have described Slammer and really appreciate your response. I cannot believe the rough shifting for a merc. I changed transmission fluid and filter twice to no luck. Changed over to a reco transmission and the same problem persists.

I haven't attempted to change the brake booster seal yet (probably a stretch for me) but placing a temporary piece of plastic with a zip tie over the boot seems to have made a noticeable difference on hissing and shifting - hopefully not just placebo as fuel consumption has dropped.

I’ve had a few transmission ‘specialists’ suggest that the gear shifts should not be rough in a merc and given the intermittent nature could not help but think it could be a vacuum leak of sorts.

Now need to adjust the hand brake slack - never ending saga with this car 😆

Welcome to the forum. You need to update your profile to say what year and model vehicle you have.
Yes it seems there's always lots of little things to take care of. As far as the e-brake goes that's pretty easy there's just a cover under the seat and you will find the cable adjustment. Super easy.
Most problems are fairly easy to fix.

Have updated my profile. Hope that has worked.

Hand brake fixed but 1-2 hard shift continues. Grok tells me with “”99% certainty” that it’s the conductor plate that needs to be replaced. Apparently the speed sensors are the issue.“Why it happens on virtually every 2004–2008 W203 (and W211, etc.) around 140,000–250,000 km:

The two speed sensors (n2 and n3) inside the conductor plate are magnetoresistive. With age and heat cycles the tiny solder joints or the sensors themselves develop micro-cracks. When cold, the resistance is wrong → the TCM thinks the input or output shaft speed is incorrect → it raises line pressure dramatically for “safety” → very harsh 1-2 shift (and sometimes 4-5, but 1-2 is always the worst).

Once the transmission warms up, the metal expands a tiny amount, the cracked joint makes contact again, resistance returns to normal, and the TCM sees correct speeds → shifts become buttery smooth and no codes are stored (that’s why you have no warning light).”

Makes sense. Is the conductor plate something you’ve contemplated changing? I may try even if I have to buy some tools as mechanics here in Melbourne for European cars cost an arm and a leg!