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.

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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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!

After I last posted, I ended up installing the new MC I ordered and I did not install the JY booster/MC assembly as the booster was a different model and I already had a new MC on hand.

The car has been parked in my driveway under a cover for the last 4 months. Yesterday it was warm and I removed the cover and opened the hood. Sure enough, no fluid in the reservoir, big puddle under the booster. I removed everything from the car. There was oil in the booster obviously.

Here's what I noticed/understood/know:

-The MC is bad because the seal allows fluid to be pulled in the booster
-The booster leaks the fluid from the bottom, hence the booster is also defective
-The booster still held a small amount of vacuum even if the car was parked for 4 months and the brakes always felt OK before that, therefore it's probably a very slow leak
-What causes the MC to fail repeatedly like that??

I will now order a new MC. I found new/reman boosters on ebay, but they are the small diameter, long type and I have the large diameter, short type (003 430 42 30). My only option with the current booster is to get it rebuild. Could I use a booster from a W202? I think I can get a reman or new one.

I used Centric brand MC's both times. The first time it broke I tought i got a defective part, but now it would be one hell of a coincidence. What could cause my MC to fail 3 times in a row like that? It's like the engine is pulling too much vacuum and blowing the seal.

Does the vacuum line appear to be oily on the inside? If so, check the oil that you find in the booster to see if it's engine oil getting in that way.

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There is no doubt it's brake fluid. The line is dry and the check valve is new. Also, inside the booster it definetly smells like brake fluid. The fluid found under the booster smells and feels like it and all the paint lifted in that area.