With an Internal CSC the fluid will always get dirty, The CSC will collect clutch dust on the sealing surface of the slave, Once it is actuated the clutch dust will smear on the inner tube of the sealing area, The fluid will clean it back off but will get in the fluid when it does, It then floats to the top of the system (reservoir) It's not something to really worry too much about but we do recommend flushing it about 2 times a year to prevent it from getting too dirty.

 

Stupid question, but what are the symptoms of a bad CMC? CSC? How do you differentiate which one is bad?

 

CSC Slave Cylinder will leak fluid and your fluid level will drop and no pedal.
If it is just the Clutch Master Cylinder you will not normally lose fluid but will not have a full pedal or no pedal at all.

Normally master goes bad from contaminated fluid. Really is best to replace both at the same time.