power steering fuild was put in my brake fluid reserve

Time to have that flushed out. Be sure the mechanic knows what has happened. Brakes are too important to take chances.

This could be a very expensive thing. Flushing may not decontaminate the brake system and many parts may need replacement. Many mechanics would be fearful of releasing a car with just flushing being the only technique employed.

Yes, it is possible that a flush may not be sufficient.
Be prepared for possibly having to replace the master cylinder, flexible brake lines, and calipers–and possibly some other components.

Have a credit card handy, and don’t ever let the person who screwed up your car get under the hood again.

Who did it? How much fluid did they put in? They could be liable for $2,000 in all new brake system so I hope they have insurance for this kind of mistake.