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Old 09-24-2017, 05:43 PM
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Join Date: Jan 2015
Location: Montreal, Canada
Posts: 54
190E leak from under or behind brake booster?

About 1 month or 2 ago, I replaced the master cylinder on my 190E because it was leaking. When I pulled it, I saw that the booster had oil at the bottom, so I pumped all the oil out and installed the new MC.

I bench-bled the MC but I made a mistake when filling the reservoir on the car. At first I filled it with fresh fluid and tried bleeding the brakes. It took me some time to figure out the thing with the rear reservoir so I eventually ran out of fluid. The next day, I grabbed the wrong bottle from the shelf and poured that into the reservoir. I think it was just old brake fluid in that bottle, but there might have been a little bit of engine oil. I immediately realized it and drained the reservoir dry. I couldn't get the rear compartment though. Hopefully the bad oil didn't reach the rear compartment.

After that there was still oil leaking under the booster. I thought it might have been oil I couldn't reach when I wiped the area clean that just found it's way there, so i cleaned it off. Today I found out the brake fluid level dropped a little bit and saw more oil at the bottom. Even the rubber hole plug is gone (where can I get one?). Did I wreck my new MC or is there a possible issue with the booster? If I buy another MC I want to make sure it is really the problem and not something else. Thank you.
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Old 09-24-2017, 06:06 PM
Diseasel300's Avatar
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Join Date: May 2016
Location: Texas Hill Country
Posts: 2,841
The brake booster should never have fluid in it unless the master cylinder is leaking. It's always possible for the fluid from the previous leaking master cylinder to still be leaking out, however since you notice the fluid level dropping in your reservoir, I'd suspect the new MC as your likely culprit.

Oil doesn't belong in the brake circuit ever. It can cause problems with rubber seals and can wreak havoc on ABS pumps.

The oil-contaminated MC should have never been installed on the car without a teardown, flush, and thorough cleaning.

Unmarked containers of fluids have no place in anyone's garage. Buy fresh brake fluid before doing the job and you have no chance of mixing fluids or contaminating the system.
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Old 10-07-2017, 02:51 PM
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Join Date: Jan 2015
Location: Montreal, Canada
Posts: 54
OK lesson learned. I won't leave old cans of brake fluid around.

Now, I just removed the old MC. I got a new MC and a complete MC/reservoir/booster kit from the junkyard. I just found out that the JY booster is slightly smaller in diameter and that the MC attached to it has 3 lines connected to it while the one on the car has 2 lines and the extra port plugged. It must be from an earlier model...

Against all odds, I didn't find swollen seals on the MC that was on the car. However, it was stuck to the booster with vacuum. I could pull on it to remove, but it would get sucked back. I had to undo the vacuum line connection to pressurize the booster to free it. I changed the check valve last time, arrow pointed towards the engine.

The booster has oil in it. It holds vacuum but it seems to leak under it. I'll remove it to inspect closer. How can I make sure it's good? Last time I drove the car, the brakes felt good, but on the few last stops they started pulsing? I doesn't seem like it's related? No external leak around the MC was visible... I'm really puzzled here!
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