What are the early signs that will tell you when its time to change the brake master?

I recently changed my brake fluid, now the new fluid is starting to look used. Is this the first sign that the brake master is going?

 

Nope. The fluid is just circulating.

Times to change the master:

1.) Leaking - Fairly obvious. I shouldn't have to explain it.
2.) Squishy, Spongy brakes - Bleed and adjjust drum brakes if equipped. No sense in replacing a master if it's not needed.
3.) Fluid doesn't circulate - Brake cylinder has two ports from the resivour to the cylinder(s)... One in front of, and one behind the piston... Push the piston forward and some fluid come up and out the front port till the piston passes it, meanwhile sucking fluid down the port behind it. Once the piston passes the front port, it'll suck fluid down both ports... As you lift your foot, some fluid will go around the piston seal to in front of the piston (It's a one way seal.), some will move back up into the resivour... If those ports are blocked. More will go in front of the piston, and the brakes will stay on for longer than necessary.
4.) When replacing the booster, it's sometimes a good idea to replace the master as well.

So while the fluid is starting to look dark, it's not a dead on obvious sign that the master needs replaced. But it may be a good idea to set aside cash and time to replace it, just in case... Masters going bad will usually either leak or you'll feel it in the brakes.

My suggestion is to reflush the brake system. Use a turkey baster to suck up all the fluid in the resivour, then refill it with clean fluid. Bleed the brakes till it comes out clear to all calipers/wheel cylinders.

 

So does this mean that the brake master failing will not happen all of the sudden like the clutch, e.g. the pedal works fine, then one day it just goes to the floor? Will I have some warning that a repair is needed and not just be stranded by the side of the road miles from home?

 

My brake master going out was gradual. Had to pump the pedal more and more to get fluid/brake pressure. Got to a point where I had to park and fix her.

 

Same here.

 

I believe that the Master cylinder is divided into two separate circuits, one for front brakes and one for rear. A sudden, complete failure of either would still leave you with some brakes. This is done for obvious safety reasons.

Another sign of a dying MC is while holding the pedal lightly (say, at a light or something) the pedal slowly moves toward the floor. This is due to the fluid leaking past the seals.