hydraulic clutch

My son has an 89 Prelude. He was having trouble getting it into gear. We checked the master cylinder and it was completely dry. We filled the master cylinder then bled the system and it seemed to take care of the problem for a while. Now the problem comes back and he bleeds the system and it resolved for a while. Could this be a problem with the slave cylinder? There don't appear to be any leaks. The clutch was replaced around 18 months ago.

Thanks in advance.