What is the outcome of accidentally adding two tablespoons of power steering fluid to the brake fluid reservoir?

Drain and refill it. Brake fluid has high compressibility, power steering fluid does not. If you haven't used the brakes at all, you can just drain the master cylinder and refill. If you've already used the brakes, may need to bleed then refill.

Good call KenF. Brake fluid has certain specifications that make it unique. Contamination concerns is another factor that support flushing the system. Please click on "Mark helpful" if you're happy with the respond. Good luck :-)

Fluids are not compressible. My concern would be what the Lucas crap does to the seals in the master cylinder and the rest of the system. I would get the brake fluid drained and refilled. If you haven't driven the car you could use a turkey baster or similar and remove all the brake fluid from the master cylinder and refill. I would still flush the brake system to be sure all the contaminating fluid is out of the system.

Mixing a semi-synthetic ATF+4 transmission oil with the brake fluid will not have swelled the master cylinder seals rapidly or anything else terribly interesting like that. The problems are more along the lines of incompatible additives neutralizing other and degrading each other;’s main component and making a gummy mess that fouls up your ABS, rear proportioning valve,, doesn’t return from the wheel cylinders causing dragging brakes ... and the biggest problem is that in the calipers the transmission oil will break down at high temperature causing more dragging... It has to be suctioned from the reservoir, then a generous volume drained through each wheel bleeder, each wheel cylinder needs to be purged a couple of times, (compress with bleeder open, close bleeder pump brake, repeat. and and then when it seems like it should be fully purged, you need an ABS capable tool to run the ABS pump to purge each wheel one more time. All this purging should consume more than just 1 quart of brake fluid.