After the fiasco with my rear brakes, I noticed that the brake fluid in the Master Cylinder Reservoir looks pretty gunky, so I am preparing to do a full flush. However, the degraded condition of the fluid has me questioning if my Master Cylinder may be damaged in some way, as I have read around the 'net that seals and whatnot can get gunked up with whatever is making the brake fluid look nasty and lead to premature failure. I don't have any problems braking at the moment, so I'm hesitant to replace it needlessly, but I'm thinking a clean start might be the best thing. I don't know the history of the car's maintenance before I bought it 3 years ago, so it may have been replaced shortly before I bought it, or it could be the original, who knows?

TL;DR
Is there any way to diagnose the health of the Master Cylinder without having to tear it apart? Or should I replace it anyway, as it is likely to have been damaged by old brake fluid debris?

 

The test I always use is the light pressure on the pedal test. Apply light pressure on the pedal with your foot and hold it there. If the pedal slowly sinks down then fluid is leaking past the seals. A hard pressure pushes out on the seals and expands them which improves their sealing. If the seals are worn the light pressure will not expand the seals thus the test.

 

It's a cheap enough part to replace, easy labor too... If in doubt, just do it.