What should you do if you accidentally poured steering fluid instead of brake fluid in the brake reservoir?

DO NOT get it flushed. A flush will circulate the oil (power steering fluid) throughout the system causing an unsafe condition that is expensive to repair. If the oil hasn't circulated in the system, you can simply replace the master cylinder and reservoir and bleed the system.

If the level of brake fluid in the reservoir was very low when you added the fluid and you have already pressed the brake pedal, it's a bit late now, proceed as if you've flushed the system. If you've flushed the system:

Replace the entire brake hydraulic system including master cylinder and reservoir and all proportioning valve brake lines (yes, steel lines included) and hoses and calipers and wheel cylinders and hydraulic control unit and/or pump if equipped with ABS. Then bleed the system.

Oil in the brake hydraulic system will destroy seals intended for use with diethylene glycol and cause them to leak. Results can be a slow or sudden rapid loss of fluid. In either case, you can get a surprise of no brake pedal can't stop car type event while driving. In the future, if you need to add a fluid to your brake system to get home, use (non-premixed) antifreeze if available, otherwise antifreeze that has a little water in it can be used. Failing that, use water. If water is also unavailable, use washer fluid. Washer fluid being unavailable, use urine. No urine? Try beer. No beer? Why not? Any of these fluids can be safely flushed out of the system requiring no parts replacement.

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== == == == == == This may be a major expensive screw up. Get it flushed immediatly and hope damage is limited. I would assume if you pull off the reservior cap that the seal is pretty fat and distorted. This will happen throughout the hydrolic brake system causing multiple problems if not addressed ASAP.