You will get a few reasons the clutch Hydraulic system is hard to bleed, yet In my opinion it is because.
Once installed the clutch master cylinder and line going to the slave cylinder has places where air will accumulate.
Here is a link that helps explain the issue and how to bleed the system properly.
There are other ways to do this yet I feel this is the only method I would use, when replacing the master cylinder or any clutch component.
I say this because this video shows you how to check for proper clutch throw out bearing travel and clutch disk release.
Once your done you will know for sure all the air is released.
Even if you do not replace the master cylinder you cannot know for sure no one else had this system apart and did the proper job purging the air.