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It's been a long time since I got on here.. Im having issues with my 02 civic clutch system..some days ago I noticed that my clutch pedal will stuck to the floor at times so it will be some issues to put into gear. I recently changed both master and slave cylinder less than 6 months and now I'm having this issue again. I noticed that the fluid in the line is dark like black so that tells me that the master cylinder maybe the issue. I changed the master cylinder and still no difference so I checked the slave cylinder and I noticed some fluid under the bottom of if so I just replaced it and an hour now, but no pressure in the line and I'm not seeing any signs of air bubbles either after pumping for so long. I even try gravity bleeding it and no fluid dripping out.
What could causing this? Some help please.

Oh, I didn't do bench bleed because I don't have the tools and the setup for that.