The fluid reservoirs for brakes and other systems are open to the atmosphere. If you look at the caps, you will see a small opening in it. Unless you have a pressurized system (like from your fuel pump to your injectors) you must have it open to outside pressure, otherwise as your pads wear new fluid will not enter the system to keep it full and operating properly. The opening is what allows your normal contamination to happen as outside air and moisture enters. If you look at a car that hasn't been maintained, you will see the brake fluid in the reservoir is a dark brown color, that is from the moisture contamination from the air. You can get around this some by occasionally swapping out the fluid in the reservoir (my brother used to do it every time he changed his oil), use some small suction device (an ear flush bulb works well) to suck out the old fluid and put in new. As was mentioned before, you should also completely bleed your system periodically to flush it out. I'm not too sure about power steering systems as I don't have much experience with them, but it couldn't hurt to at least change out the fluid in the reservior too.