the master cylinder or the slave cylinder is shot. my slave cylinder was completely shot and i had to fight it to put it into gear after pumping the clutch many times when i lost pressure. even though it wasn't visually leaking oil, it was a bad part. you mention that the slave has been replaced? maybe its as simple as the clutch master cylinder? its an easy do it yourself swap and the part will be less than $50. good luck bro.