Just to be clear, there is no such thing as a "brake/steering" fluid system in your car. It's either brake fluid or steering fluid - these are two separate systems with separate reservoirs and separate fluids.
Some vehicles (including my Chevy Astro) use the power steering pump for the power assist brakes. Most vehicles use vacuum for power assist, but not all. I found this out the hard way when my power steering pump died. I couldn't brake or steer. Luckily, I was parked when it died. There is still a separate reservoir for brake fluid. I think the OP was confused about what fluid was being replenished.