I recently purchased a 90 535i for my wife and have been methodically replacing worn items. It started witht the drive shaft and wind shield and has moved on to now needing a headliner and possibly a master cylinder ( and of course new Conforti chips, Yoki tires, Bilsteins, etc....I have to disguise the upgrades in with the replacement of worn items to keep the purse holder in agreement with expenditures.)
Anyhow, I heard rumor and read from a website something about the rear seals failing due to a design or manufacturing flaw on the original E34 factory master cylinders with ABS units, causing them to leak slowly down the back on the booster cannister. I remember something being said to the effect that this was more of a nuisance and annoyance than a serious issue of braking capacity or catastrophic failure. (Though I must say that a leak anywhere in a brake system tends to make me a bit apprehensive, especially since my wife is the primary diver.)
I had noticed the paint bubbled and some paint missing from the booster when my friend, whom owns and runs a local BMW certified shop inspected the car before purchase. The entire unit and surrounding areas were dry and the previous (female) owner was unsure if it had been replaced. The MC looked new and had fresh metal color...thus we assumed it had been recently replaced.
Yesterday I discovered the fluid has slowly decreased and the bottom backside of the MC where it attaches to the booster wet and the booster is wet from there down. I could find no leakage on the inside of the car near the cylinder to pedal pass through. (I 've had previous big four models saturate my carpets and floor mats when the MC craps out.) I checked the fitting connections from the ABS and they're tight and dry.
Has anyone out there heard or know of a specific flaw in the original factory master cylinder seals causing this type of leak?
My guess is it has just worn down the inner piston seals or someone may have added the wrong grade or type brake fluid at some point causing the seals to degrade prematurely. None of the calipers show signs of leakage and the brakes feel to be working quite well.
I guess I shouldn't forget to mention the car has 120k of use, which when compared to my '82 320i with 250k is just past the break in period.
Any info, experience or advice on this is welcome.