Brake fluid absorbs water over time, and as it does this, it turns black. If you let it go too long the water content will reach a point that it will start to corrode the inside of your brake parts, and seals will fail. I try to swap my brake & clutch fluids every 2 years.
My preferred method is to use my pressure bleeder. What I like to do is suck out all the fluid that I can from the reservoir with a turkey baster, then I put the pressure bleeder on and blow air thru the entire system until nothing but air comes out the bleeders. This helps get all the old fluid out, although there will still be some trapped in the calipers. If you really want to be thorough you should pull the calipers off and invert them, drain out all the fluid.
Now that the system is empty, pull all the bleeders off, clean all the threads and anti-sieze them. Carefully inspect all the brake lines and bleeders, anything that is rusty or cracked should get replaced. Reinstall the bleeders, close up the system, then start bleeding new fluid in.
In order to get the master cylinder fully bled (all the air out), I just presurize and de-pressurize the pressure bleeder a few times, you can watch the air bubbles come out of the MC up into the reservoir. When this stops happening, you're good to pressure bleed the rest of the vehicle.
Start at the farthest away bleeder (LR wheel), open the bleeder with a hose on it going into a soda bottle, you'll be surprised how long it takes before flesh clean fluid comes out. Let it flow till it looks nice and clear and all the bubbles are gone, then move on to the next one (RR, then RF, then LF, LSPV last if you have one). It will take at least a quart of fluid to do the whole vehicle, more if you're doing new or empty calipers. I tend to use a lot of fluid when I do this, but my thinking is brake fluid is cheap, good brakes aren't.
I like the Prestone Synthetic fluid (NOT solicone DOT5).
