Mike,
Recently I "topped off" my master brake cylinder with power steering fluid (accidently, of course) adding about 2 oz. to the original brake fluid. What will happen if I leave the power steering fluid in there? Is this a serious enough mistake to drain the system, or can I let it slide?

SUCK all the fluid out of the master cylinder NOW! Disconnect the brake lines off the master cylinder and flush it with new brake fluid or flushing fluid for brakes. Watch the fluid coming out of the line holes while someone else pumps the pedal, it should run clear. If the oil didn't get into the master cylinder and was only in the reservoir you should be OK. If it got into any of the rubber parts in the system they will fail as they are rubber and not oil resistant. Be very aware of the way the pedal feels, if it goes down real slow by holding pressure against it you will have to at least replace the master cylinder. Good Luck.

Mike