I wonder if anyone can help me. If my brake fluid reservoir was almost full of brake fluid and I (1) accidentally put some (not much because there was not much room) oil (DW 40, not synthetic) in that brake fluid reservoir (instead of where the oil belongs) and (2) then drove about a quarter mile before (3) removing (with a turkey baster type device) everything from the brake reservoir and (4) finally refilled the reservoir with new brake fluid, am I in any imminent danger? The oil is lighter than brake fluid (I tested the two in glass to determine this). Opinions on the web range from “don’t drive your car and it is going to cost you a fortune to fix” to “it’s no big deal you’re fine.” That is a very wide range of opinion. Money is dear. I will do what I have to do to be safe. But I don’t want to act out of sheer paranoia. Thanks in advance.
It certainly is good that you got the “mix” out early,there is a ratio of motor oil and brake fluid multiplied by the time the “mix” was in the system that will cause rubber parts problems,have you reached that number? I say no, but I have no testing to back it up.
I have seen major “mixes” cause tremendous problems but your situation does not approach those circumstances.
I would do a flush and explain to the flusher whats up.
Chances are, no contaminated fluid reached the rubber parts in the master cylinder and you should have no further problems. But since this is a critical part (the master cylinder) and today’s hungry lawyers leave no stone unturned, replacing the master cylinder would be cheap insurance. In all likelihood, the motor oil will have no negative effect on the rubber parts, there are a lot of urban legend theories about this, but why take a chance now or in the future?
Since you added only a small amount of oil and managed to siphon some out odds are you will be fine.
Motor oil is bad for the rubber parts in the brake system. While rebuilding the hydraulics on a clutch system one time (same principle, same brake fluid) I was pulled off of the job for a while. When I went back to it I had forgotten that I had washed the grease off of my hands in the solvent tank; which uses a solvent called mineral spirits.
After putting the clutch master cylinder together and trying it out I disocovered that it worked fine; for about 10 minutes. After that, nothing at all.
When disassembled I realized that my handling of the rubber parts had ruined them because the traces of mineral spirits on my hands not only swelled the rubber parts up but also softened them to a foam like consistency.
Since mineral spirits is a petroleum distillate like motor oil and gasoline, I’m of the opinion that what you did could possibly be harmful to the brake system.
You should be ok considering the small amount involved but I would flush the brake fluid just to be sure.
That’s a lot cheaper than replacing brake hydraulics; especially on ABS cars.
(And yes, I learned a lesson on that one. Always wash the hands with soap/water before handling any clutch or brake hydraulic parts.)
Thanks for your answers, which seem fairly consistent. I will try to avoid driving today, Christmas, and use my girlfriend’s sister’s car to travel to family, if possible. If I can’t do that, I may have to drive (I’ll stick to the right hand lane and try to drive like the old people who usually annoy me so much). But in any event I know of a garage open on the 26th and I’ll take my car in then.
Though your story about the clutch repair is worrisome. I take it the solvency of the mineral spirits is what caused the problem, and that motor oil ranks lower than mineral spirits in that respect. When I looked this up I learned that mineral spirits ranks a (30.7), mineral oil (2.6), SAE 10 motor oil a ( 2.6). Though I don’t know what kind of scale they were using to measure this.
Later on I actually tested a few rubber parts with mineral spirits just to verify with 100% certainty what had occurred. I put a few drops of mineral spirits on my hands, rubbed them together, and then wiped my hands with a clean dry rag.
I handled a couple of rubber seals, sat them on the bench, and about half an hour later they were about 20% larger than they were originally.
I think you’ll be fine and congratulations to you for actually thinking about the problem and doing something about it.
Most people would have turned the radio up a few notches… 
Well dying with the radio on is the way to go. Thanks to you all for helping me on Christmas Eve of all days!!! All appears to be well after driving for about an hour and a half. I wonder if FLUSHING the brake fluid might force any residual oil still hiding in the reservoir through the system and thus do more harm than good. I wonder if just overfilling the reservoir will more safely purge any oil residue (which floats on top of brake fluid) out the top. Anyway, I emptied and refilled the brake reservoir a second time from sheer paranoia. It smells like brake fluid, not like oil and I saw no oil floating in the reservoir. I think the oil is all gone.
And thanks for repeating the mineral spirits test!!! I’m going to buy a couple of rubber seals, put DW 40 on my hands, wipe it off, and then handle the seals and see what gives! Though I don’t think any oil got to any rubber-like material in my car. But it will be a fun experiment anyway.
Did anything ever happen to the brakes?
Same thing happened to me, just wondering what your outcome was.
Any long term update from anyone else who had this issue/situation?