Accidentally put engine oil in brake fluid reserve


so i know it sounds dumb. But i accidentally out engine oil in my brake fluid reservoir. i just bought the car and they are close together and the reservoir cap is heavy duty and kinda looks like an oil cap. anyways im dumb blonde okay. it was only about a quarter qt before i realized what i was doing accidentally and i stopped. then i drove the car home still, so i drove for about 30 miles total. ive read other threads saying im screwed cause oil and rubber bad together blah blah. this only happened a couple hours ago is it still possible to flush the systems or anything besides a full on replacement of the systems?? i am poor.

Accidentally put engine oil on brake hydraulic - General Discussion - Car Talk Community

It must be flushed out immediately if you want to prevent damage. Even then, you may still suffer damage at a later date. Be sure and tell the shop what you did. They may also ask why the brake reservoir was so low to begin with. That in itself may indicate another problem.

If I may ask, if you are so poor, why do you own a BMW X3? If you tell me you bought it cheap, I’ll tell you a cheap BMW may be the most expensive car you’ll ever own.

Even the brake fluid flush won’t be cheap on a BMW. If you don’t do it right away, the repair bill you’ll have later will make your credit card melt.


You’re going to be a lot poorer after the BMW dealer’s service department gets through with you. Putting the oil in was your first mistake. Driving the car afterwards was your second. Have the car towed to the dealer, tell them what you did. See what they say. If they start off wanting to change everything in the brake system, ask them if they can do a complete flush of the system first and see if it works.


+1 to this advice, yet I fail to see the reason in using the dealer (the most expensive option), unless car is off the warranty… and it looks like it is…

I would ask any competent local mechanic to tow it directly to their shop and to make the flush, probably repeatedly, and hope for the best


Because I don’t know if the OP lives in a one stoplight town in the middle of nowhere where the local mechanic has never worked on anything more complex than a Chevy Cruze.

Seen so many put the wrong fluid on this board, don’t beat yourself up too bad. It happens. I do not know if oil will float on brake fluid and you could at least use a turkey baster to pull of what you can.

You don’t have to take it to the dealer to have the brake system flushed

Any shop with the proper equipment can perform this service.



Concur w/advice above, most important priority is to not drive it , or even step on the brake pedal. Have it towed to a shop (doesn’t have to be a dealership). They’ll probably first use a turkey baster to remove everything in the brake reservoir. I think if I had this problem myself I’d try to use a vacuum device to clean out the brake lines, sort of vacuuming the lines one by one from the top. My vehicles don’t have ABS, and yours probably does, so that might affect whether the vacuum removal method is a good idea or not.

This effort may fail btw, and all of the brake system’s hydraulic components will require replacement.

You didn’t do this b/c you are dumb. You just didn’t know is all. Suggest to pay someone knowledgeable to show you how to do the basic maintenance stuff your car requires. Then you’ll be able to do it yourself going forward. Asking here before you try something new is a good idea too.

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I would assume that the engine oil has not travelled far from the master cylinder…unless the two fluids intermix. The turkey baster followed by disconnecting the brake lines at the corners might be better than running the bad oil through all the calipers.

too late for that:

The OP is going to need to have the car towed to a shop to have the brake system flushed out and checked

Brake fluid is heavier than petroleum-based fluids and does not automatically mix with them.

The oil is just sitting on top of the brake fluid.

And since the brake system isn’t a circulating system but instead a displacing system, it’s pretty much impossible for the oil to mix with the brake fluid.

So the OP should drive to nearest shop and have the system flushed.



Over the years I have had several vehicles show up with brake problems and found that the master cylinder cap seal was swollen and spongy and the master cylinder was totally defunct. And after replacing the master cylinder on those vehicles and finding a firm pedal and full brake function my best memory is that all returned within days with other failures resulting from the contamination. Once the brakes have been applied a few times it seems certain that the fluid moves downward into the system and destroys any rubber it contacts. I learned to write out a detailed disclaimer and denial of any warranty on all brake repairs when there was evidence of fluid contamination and it was a relief when customers took their vehicles elsewhere due to my refusal to warranty such a repair.

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