Mysterious break issue - Page 2 - Mercedes-Benz Forum
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post #11 of 16 (permalink) Old 01-05-2019, 05:15 PM
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If you by any chance adjusted the brake peddle/push rod when you were trouble shooting.. it needs to be put back to it's original position
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post #12 of 16 (permalink) Old 01-05-2019, 08:07 PM
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Did the new master cylinder come with a new vacuum seal ?
A new vacuum seal must be fitted every time the mc is removed.
Sorry if this is a stupid question but have you started the engine up to generate vacuum ?
Also ,the spring nut thread on the end of the master cylinder should be about 1/4 inch from the end of the mc.

Last edited by Benzyle; 01-05-2019 at 09:24 PM.
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post #13 of 16 (permalink) Old 01-06-2019, 05:04 AM Thread Starter
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@bobby735 - we did not adjust the rod.
@Benzyle -
- I did not see a new vacuum seal. Is that something I should have received and where does sit go?
- Yes, the engine was running. The vacuum pump is pulling a vacuum.
- Regarding the spring nut, I actually don't know where it was because I had my sons put the master cylinder on and bleed the brakes while I was at the hospital with my mother. When I got home, they were finishing up. That's when I noticed the issue. I worked on it for a couple hours with no luck. My son is a truck diesel mechanic but doesn't have any experience with Mercedes brakes and master cylinders. However I saw putting on the master cylinder a pretty simple job and didn't give it too much thought. A problem I noticed while watching the car run that was brand new was a small oil leak coming from behind the engine. There was a small hose on the vacuum pump that had come off and was spraying small amounts of oil out. It almost seemed like a vent on the vacuum pump and this small hose was under vacuum sucking excess oil out of the pump (just guessing) but I put the hose back on (pain the the butt to get to). I have no idea why that hose came off but it wasn't like that before. I've been wondering if my boys somehow caused that to occur by pumping the brakes without the engine running and pressurized the vacuum pump and blew that hose off.
- Let me share some more as background... the master cylinder I purchased was not OEM. I purchased it from AutohauAZ where I often buy parts and was a Metelli. When it came, it had 3 threaded ports and my original had only 2. The 3rd port on the new one was not a pressure port and seemed like it might be a return port for some cars. I called Autohaus and verified that the master cylinder was the right one for the car. They confirmed it was and said that all the ones they had had teh 3ed port and that I needed to plug the 3rd one if I didn't needed it. So I did.

Ok, that's a bunch of info. When I read this thread at 4am this morning, I thought I saw more content that has now been deleted - something about must be OEM master cylinder and some other stuff - and then when I went to write this, it wasn't there any more. So I don't know if I accidently deleted it between when I read it earlier till now. In any case, thank you guys for supporting me.... my mom is still in the hospital and I'm headed there after I finish this; between that and car problems (I have another Mercedes, e320 CDI with an issue also that I'm trying to work on at the same time) I'm running on empty!!
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post #14 of 16 (permalink) Old 01-06-2019, 12:39 PM
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The vacuum seal is an o ring that goes around the body of the cylinder , just inside the flange where it bolts up to the booster.
It is critical to maintain a vacuum , so if you don't have one you will not get any vacuum maintained in the booster and so would give you a hard pedal.
I would expect it should have come with one ,,,but you never know .
As I said , a new seal must be fitted every time the mc is removed.
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post #15 of 16 (permalink) Old 01-06-2019, 01:33 PM Thread Starter
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Over the past hour, I pulled the master cylinder off to see what might be wrong. For reference purposes, there is no adjustment on this particular car that I can detect. It's just a rod that comes out with no adjusting nut. We also checked the depth of the cylinder of the old one to the new and they are exact. THEN I noticed the missing seal.... yep, the vacuum seal. My son had not seen that when he put the new one on and the new cylinder did not come with one, so he overlooked it. A good lesson for him. We put it back together and bled all the brake lines and she seems to be working fine, at least for the moment. As I started this thread, it was a mystery to start with from what my wife told me so I'm going to hope and pray that the master cylinder corrected the issue.

Thanks guys for your willingness to help, much appreciated!!
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post #16 of 16 (permalink) Old 02-25-2019, 01:13 PM
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Mysterious break issue

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Originally Posted by mdelfunt View Post
As I started this thread, it was a mystery to start with from what my wife told me so I'm going to hope and pray that the master cylinder corrected the issue.

Thanks guys for your willingness to help, much appreciated!!
Did the replacement of mc solved the issue?. Reason I ask is because I may have a similar problem. The wife drives an 05 C230K, she called me from work panicked that when she arrived at work she pushed the brakes to stop and she could not push the brake pedal, the car kept moving. Apparently the brake pedal is so stiff that even with her entire body weight she couldn't push it with her foot. Luckily this happened in the parking lot at work.

I can only think of couple of things that can cause this behavior, the brake booster hose coming from the engine or the brake booster.
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