What's the internal barrel like? Smooth and unmarked or perhaps scratched and or stepped?
Paint or plastic coating not a great idea especially if it can fail! I must admit I have never seen that before. Dirty fluid is very common in a clutch as it holds a small volume and gets used 1000's of times more than the brakes! That micro graining then wears the seals and believe it or not the barrel too. fortunately a clutch with an external slave is usually easy to bleed so a regular bleed is recommended. I tend to change clutch fluid on colour rather than time. looks in this case like the debris had started to jam the piston and that was why the piston was hanging up in the barrel, on sudden release it would knock and feel like the clutch was snatching.
When it comes too the concentric clutches, well that's an whole other ball game. If it fails then that's a gearbox out job and when bleeding they are very susceptible to damage. so real caution needs to be applied if pressure bleeding them. Personally I like the back bleeding technique as you have full control and can also empty the reservoir first and fill as you feed from the bleed nipple. some concentric slaves do not have access to a bleed nipple and the ones that do, will require extreme care when loosening and tightening them or setting the stop to the correct position as they are quite fragile and often not supported where they exit the gearbox. If you really don't want to go there just syringe out all the old fluid from the reservoir and refill with new clean fluid of the correct grade. Dot 3 and 4 are compatible with each other, 4 has a higher temp rating 5.1 is a silicone based variant and don't ever mix them or use it in a 3 of 4 system that has previously been used.
