Oil in the brake reservoir

Last night I mistakenly poured a small amount (less than 1/4 cup, approx) of synthetic motor oil (5w30) into the power brake reservoir of my 2008 Cadillac STS. It was too late for a garage to be open, and I was about 80 miles from home. I tried to scoop as much of the oil as I could out of the reservoir with my fingers, then drove it home with no problems. Other than leaving all future maintenance to the pros, is there anything else I should do now?

Oil can cause damage to the brake system. Hopefully you managed to scoop out enough that nothing bad happened. However, you really should do a full brake fluid flush at this point.

The entire brake system is now contaminated and WILL eventually suffer catastrophic failure. Oil even in minute quantities will attack the seals in the master cylinder and cause failure.

Have it towed to a trusted mechanic or a dealership. Unfortunately the fix will be expensive. This is what Tom and Ray recommend:


At a minimum, you should have the master cylinder replaced and the brake lines flushed.

I would have the brake master cylinder replaced today and have the entire brake hydraulic system flushed out. And for the future any irregular brake problems will likely be caused by the contamination in the system now.

I tried to scoop as much of the oil as I could out of the reservoir with my fingers, then drove it home with no problems.

If you got it nearly emptied then I’d say there’s a very good chance you’ll get away with it entirely, but when dealing with brakes you want to be careful.

^ OP said she tried to scoop the oil out with her fingers (!) then drove 80 miles. No way there’s any chance she’ll “get away with it entirely.” Trying to do so is asking for disaster. The master cylinder is thoroughly contaminated and dangerous to drive.

Oil does seem to float on brake fluid until it is stirred up and after scooping out most of it if the master cylinder is replaced and the system thoroughly flushed ASAP as asemaster advised there is some hope that there won’t be any catastrophic failure of components further down in the $ystem. If the oil is left in contact with rubber in the brake system it will emulsify it and result in some very expensive repairs. I believe we have seen some awful stories on this forum regarding mistakes topping off brake master cylinders.

Would it better damage control to disconnect the brake lines into the ABS controller and flush out those lines; also disconnect the brake lines at each brake and flush them? Idea being, it’s highly unlikely the oil got that far down, but take care to avoid bringing any of it to the downstream rubber parts.

Does the ABS controller have rubber parts that are exposed to brake fluid? That should figure into these decisions.

The master cylinder does, and replacing it may be an excess of caution. Could it be disassembled, cleaned with brake cleaner, reassembled and reinstalled? If it were my car I’d do that.

Long ago I reguilt master cylinders with great success but dual piston models put an end to that in the 60s. I would strongly discourage a DIYer from opening any master cylinder with the intention of returning it to service. It would be worthwhile to disconnect the fluid line at the ABS controller then, after replacing the master cylinder flush the open line and then the enire system.

For maximum safety the recommendation would probably be to replace the entire brake system hydraulics. Master cylinder, ABS, and calipers. Probably keep the same metal tubing, but replace the rubber component of the tubing. And a very thorough flush of the metal tubing with the correct fluid.

If that situation happened to me and I discovered it before driving the car, I definitely wouldn’t drive the car. I’d take a taxi to the nearest hotel, and have the car towed to a shop, and ask that they disconnect the master cylinder asap. Then replace it with a new one and a thorough flush. If done that way, I’d expect no further problems.

If it happened to me and I was in a situation where it was mandatory to continue to drive the car I guess I’d do exactly what you did. Then back to home base I’d replace the MC, flush the system, install new brake fluid, and hope for the best. I’d be constantly aware of the brake performance and any signs of degraded braking for the next year or so. And I’d accept I was taking some safety risk. So when it came time to replace the pads and rotors, I’d probably replace he calipers too.

Flush the brake system and hope for the best.

Another thing on the to-do list is to figure out where the leak is that caused the brake fluid level to be low. My assumption, right or wrong, is that a brake warning light is what caused you to check the fluid level while on the road at night.