I just replaced my master cylinder about 3 months ago, but I think my slave went bad now. I had to remove the rubber line for the clutch temporarily, and after bleeding the crap out of it, still cannot shift at high RPM's. Everything worked/ shifted fine before. Is it mandatory to replace both the master and slave together? I did search, but couldn't find an answer.

 

I would... I did one without the other, only to find out the other failed months later

 

Try bleeding again