I've received feedback on this issue from various ppl. ranging from "it's normal" to "your entire brake system is completely rusted out and you're going to die, etc.".

I've changed both fluids several times with a vacuum pump w/proper bleeding order completely flushing both systems, and in approximately a one/two month time frame, the fluid in both MCs begin to turn brown.

Is this anything to be concerned about? Is it time to change out the hard lines?

On a theoretically new brake system (lines, MC, brake cylinders), how long would the fluid remain clear?

 

Discoloration is caused by corrosion, either of the lines internally or the wheel end parts (caliper/wheel cylinders). A dark color is caused by rubber wear. Some level of this is normal, depending on the car.

Most GM's have a very dark greenish colored brake fluid after 50,000 miles or so. I have also seen Toyota's with nearly clear fluid after 150,000 miles.

Testing the boiling point and the metal content are most accurate ways of testing brake fluid.

Simply installing new fluid in most cases will remove debris that is attached to the lines/calipers/wheel cylinders etc. This fluid will be carried by the rest of the brake system. At some level corrosion is inevitable.