Brake fluid leak between the brake booster and brake master cylinder?

[sreten]

Howdy fellas,

I had a pretty big Pentosin leak from the booster when I got the car. (1991 850i). Since then, I replaced the large O-ring on the booster and the leak from the weep hole stopped, I bled the brakes twice, went for a test drive and all was fine and the brake pedal was nice and firm.

Now I checked the weep hole again and it seems that brake fluid is leaking there. The fluid is clear in color and when I smell it, it's def brake fluid. To be sure, I compared the smell of the leaking fluid with a bottle of brake fluid and it's the same smell so it can't be Pentosin again. I should mention that I installed new grommets for the reservoir and the leak doesn't appear to be coming from above or anywhere else but the weep hole.
On top of that, the brake pedal has gone soft again, like when I got the car. Also, when the car came to me the brake fluid reservoir was low on fluid and I never did find any other leaks.

I just wanted to double check on here before pulling a trigger on a new master cylinder as this doesn't seem to be a common leak. In most cases, it's the brake booster leaking from the weep hole but in my case, everything is pointing to the master cylinder.

Thanks!

[rjbaren]

There is a possibility that the master cylinder reservoir is leaking. The seals under the reservoir in my car leaked twice. They swell up. If you feel around the weep hole it will be wet but it could be coming from above and be your reservoir. Clean everything up as best as you can, refill with brake fluid and see if it is not wet just under where the plastic meets the metal under the reservoir. If that's the case, then you have to change those two seals and you won't even have to bleed the system. remove the reservoir, and change the seals. The seals were very inexpensive. I hope that is the case.

[sreten]

Thanks for the feedback, but as I mentioned the seals/grommets I replaced when servicing the brake booster. When I feel around the brake fluid reservoir, specifically where it meets the master cylinder it's dry which is why I'm suspecting the problem is the MC.

[rjbaren]

You might want to check some master cylinder threads. I think the cylinder from a 7 series fits at a much reduced cost.

[sreten]

Yes, I found in few threads that E32 master cylinder fits, part # 34311156643 so I'll be ordering that one. It's much cheaper.

[JDP530]

I had the booster rebuilt and changed the master cyl to a new E32 '643' park last summer and brakes work great now. Before changing parts the pedal was quite soft and had too much travel but brakes still worked. Then I finally noticed a very small puddle of fluid on the floor along with fluid at the back of master cyl where it mates to the booster cyl.

[ChrisFuture8]

Hi JDP. Yes, I agree with your feedback about the brakes feel on the E31. Too soft and not as effective as other BMW platforms (unless a bunch of money is poured into upgrades). Could you expand on the E32 '643" comment? Was it a fairly straight forward plug and play?

Thanks.

[obispoil]

Hi JDP. Yes, I agree with your feedback about the brakes feel on the E31. Too soft and not as effective as other BMW platforms (unless a bunch of money is poured into upgrades). Could you expand on the E32 '643" comment? Was it a fairly straight forward plug and play?

Thanks.

it is. the cylinder diameter is 25 instead of 27 but I can't tell the difference.

[JDP530]

Hi JDP. Yes, I agree with your feedback about the brakes feel on the E31. Too soft and not as effective as other BMW platforms (unless a bunch of money is poured into upgrades). Could you expand on the E32 '643" comment? Was it a fairly straight forward plug and play?

Thanks.

Fits just fine, but you will have to use your old reservoir. The E32 reservoir is shaped differently and I don't believe it will clear the auxiliary pump and hoses. I just cleaned the old one from my car. I actually filled it with some bb's and shook it around with Seafoam, letting it soak overnight before I got it really clean. Looks like the reservoir is available from dealers for about $90.00 34331159282.

I did remove the auxiliary pump assembly out of the way to I had more room to remove master and booster.

[sreten]

Thanks for everyone's input. I order new master cylinder 34311156643 and more brake fluid. Hopefully, this solves the issue.

[rjbaren]

Does the 643 master Cylinder fit the 840 too?