What year is the bike? I had an 08 525xc before my sx and I had the slave cylinder leak all the mineral oil into the engine. I replaced with one that was brand new. But your issue could be that the line needs bled, did you bleed it after you replaced both the slave and master cylinder? The 08 supposedly had that as a common issue and in 09 they fixed it. If those are all good my next guess would be the clutch pack itself or maybe your springs?