Brake fluid is hydroscopic, that is it absorbs water over time, even inside your brake system. The moisture will cause rust and corrosion eventually and eat away seals also. I always try to flush it out every 2 years, always using a fresh bottle of fluid(because once you open it, it begins to absorb water). I've heard the synthetic fluids absorb moisture slower, but they cost more.
On the power steering system, you can use a suction gun to suck out the old fluid from the reservior, then bleed the old out of the return line after refilling the reservior with new. Power steering fluid is much like trans fluid in that it carries away metal particles from the moving parts. Both of these are often overlooked prevenative maintenance items that will make your accesories last as long as the rest of your truck.
1989 GMC S-15 4x4 4.3L/700R-4, 3.08s, 31" Sport Kings For Sale
1985 S-10 4x4 Soon to have 3504BM/TH350/D44/14FF, 35" Boggers