Likely the problem is just nasty black fluid with gritty goo. Or it least it probly started that way......Gotta replace that Mineral oil once a year or it'll do that. Then the gritty goo fails the seal on the M/C and/or Slave.
If you get to it in time, you should be OK.
Flush it all, pull the slave and take it apart to clean it all. Clean out the M/C also.....Then refill and bleed.
Many here will say ATF lasts longer than Mineral Oil.
Ride OREGON First .com
"got no problem with keeping truly roadless areas as wild....
On the other hand, if it has been logged or mined and roads already exist,,
...then that land should be open for public use" (peterman)