[E91 Hoarder]

Noticed brake fluid sipping inside on the brake pedal from the booster. Is this fixable with a seal or the whole brake booster needs replacement?

[HondaRC51]

Replace the booster and master cylinder. There's a seal between the master cylinder and booster that is leaking into the booster. I don't believe there is a rebuild kit for BMW master cylinders but I could be wrong. Either way the booster should be replaced as the diaphragm inside will deteriorate due to the fluid leaking.

[E90_slides]

Do you also experience lower mpg engine running rich? My booster is also leaking fluid into the cabin and I believe this has caused a vacuum leak. Never get a CEL and it runs well but I get a rough cold start every time and tailpipe is always black (running rich as f**k 18.5 mpg) I got everything but the booster smoke tested and they couldn’t find the source.

[Andystobbs]

How would a vacuum leak leave you running rich?

[E90_slides]

Originally Posted by Andystobbs

How would a vacuum leak leave you running rich?

It causes me to believe that the unmetered air would cause a lean condition, telling the dme to add more fuel causing a rich condition. If you have any info relevant to helping the situation it would help tremendously. Thanks 👍🏻

[E91 Hoarder]

I did not notice anything like than, but would also like to understand the connecton.

[E90_slides]

Originally Posted by BMWTF?

I did not notice anything like than, but would also like to understand the connecton.

Vacuum is continuously applied to the booster via the vacuum pump. If there is a tear, allowing unmetered air to be sucked in through the booster upon load, the air has to get emptied into the crankcase. Where else would it go??? It just doesn’t disappear. That extra crankcase pressure gets recycled through the pcv and into the intake causing a lean condition, dme reads more air at the lambda sensors, telling fuel injectors to dump fuel. Not too hard to understand I hope.