The brake booster seals on the W203 (and other MB and Volvo platforms) are known to wear out, creating a hissing noise (vacuum leak) that gets worse over time.
Of course MB doesn't sell the seal separately, since the OE brake booster cylinder is an assembly by ATE. On my car, the failed seal is ATE PN 03.7718-5018.1, made of EPDM. Searching online, I was only able to find 2 aftermarket replacement kits:
URO 0014300708S - booster seal kit, also made of EPDM
AliExpress - some red silicone version of the seal. The reviews generally looked bad so I went with URO.
A lot of the videos online show the seal installation done with the booster in a bench setting. As I really did not want to remove the booster from the car, I wanted to see if it could be done on the spot. The answer is Yes.
For the actual seal job:
13mm deep socket - a box or offset wrench can also work, but a deep socket is much faster
Cardboard - for protecting car sheet metal (will explain later)
Dead blow hammer - 2-3 lbs
Pry tools - something plastic with a flat edge around 2cm wide with a 90 degree bend in the tool (limited room)
Extensions - 3" and 6"
Thin slot screwdriver or punch tool
Silicone grease - I used Super Lube 92003 as it also contains PTFE (great for sliding applications)
Do not use any non silicone based grease, as it may swell and ruin the seal.
Rags for cleaning up the old grease
This is very similar to the the HVAC stepper motor job, so I won't go into details. Remove the panel underneath the steering wheel (the one with the headlight switch and ignition switch), as well as the panel below with the hood release. The hood release latch is held in place with a single Philips screw.
The brake pedal is covered by a plastic panel with a large foam piece in front. Remove both. The plastic panel just pulls off. It is secured by some clips, as well as 2 tabs at the top and 1 tab at the bottom.
The brake pedal is hooked to the booster with a pin and clip. Use a thin slot screwdriver and slide the clip out in the direction of the purple arrow. The pin comes out nice and easy afterwards.
Brake booster pin clip unclip direction
Clip location
The brake pedal assembly is secured to the car with 3x 13mm nuts, 2 of which also hold the brake booster in place. I used a 3" extension for the top nut, and a 6" extension for the bottom 2 (the ones holding the booster).
Cut some cardboard squares (1.5-2cm per side) and punch a hole in the middle with a sharp tool. Push the squares onto each of the 2 studs holding the booster to the car. Personally I used 2 squares per stud.
Using the 13mm deep socket, drive the 2 nuts back onto the booster.
This was by far the most difficult part of the job.
The instructions here assume that the replacement kit is the URO one, which comes with the installation tool (basically a specially cut section of pipe).
Take the silicone grease and smear a pile of it a) all over the seal inside ridges, b) all over the seal outside ridges, and c) all over the booster piston. Honestly I don't think it's possible to use too much grease. I just piled the grease everywhere, including the gap between the piston and booster.
Slide the new seal down the piston and into the booster. Now the seal has to be "tucked" into the gap all the way around. The URO instructions say to use the flat end of the pipe, but that doesn't really work outside of a bench setting, as the lip will pop out here and there. I used the "sloped" end instead, and pushed on the pipe while using the trim tool to tuck the lip in a bit at a time. Patience is key. Do not get frustrated or the lip will pop out again. Gently work your way around the circumference. As I finished each section, I also backed up and gave the "completed" areas a hard push with the pipe to make sure they didn't pop back out.
Use your finger and confirm that the lip is 100% tucked in. Once this is done, give the pipe (sloped end into the seal) a few good HARD pushes around the diameter, then use the dead blow hammer and WHACK that pipe. Do not be gentle or the seal will pop out again. Once I was sure the 1st lip was clearly in, I alternated between flat and sloped ends and whacked away until the seal was fully installed.
If you make this far, congrats.
The URO video below shows the general idea, but in a bench setting. At the end of the video it is mentioned that the seal is fully installed when the lip is flat against the booster.
Reinstall the boot, and be sure to pop the edge over the new seal. This is much easier with the brake pedal assembly removed. In other words, do this first.
Remove the 2 nuts and cardboard pieces, and reinstall the brake pedal assembly.
Put everything back together, and grab yourself a cold one.
GACK! So you did all this laying on your back in the wheel well?
Did you remove the drivers seat to give yourself more room?
Or just contort yourself sideway? Uggh!
How much of the air vent procedure did you have to do?
I really don't understand post #5.
Obviously at some point your removed these nuts to put the cardboard on but not seeing where that is stated.
Whats the cardboard for?
What did that accomplish? What exactly IS the "secret sauce"? (Snake oil haha)
GACK! So you did all this laying on your back in the wheel well?
Did you remove the drivers seat to give yourself more room?
Or just contort yourself sideway? Uggh!
How much of the air vent procedure did you have to do?
I really don't understand post #5.
Obviously at some point your removed these nuts to put the cardboard on but seeing where that is stated.
But what did that accomplish? What exactly IS the "secret sauce"? (Snake oil haha)
Yes, I did everything lying on my back. Having a short stool outside the car helped, as I could just lay flat. Not the most comfortable, but it was doable. I slid the driver's seat all the way back, but did not have to remove it.
Which air vent thing are you talking about? If you're talking about disassembling the car, you will to remove the big piece of trim below the steering wheel. Basically I had to remove the centre console as well as the trim with the radio/HVAC/switches, but not the radio itself.
The secret sauce is that by securing the booster this way, the firewall now holds it in place like a bench vise. So now I can do the entire repair like in the URO video. The cardboard pieces are there so I don't damage the paint. The deep socket is recommended because the 2 nuts now thread much deeper with the brake pedal assembly removed.
I don't actually need to do this, just playing devils advocate in case someome comes here in the future and
we're not around. Hopefully we will be. I wonder how long someone will continue to host all this content??
Hopefully for a VERY long time.
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
