Sounds like a clutch MC from here. There's a chitload of little seals and crap in there, so you don't necessarily lose fluid when the MC goes south, the fluid can just blow by the seals internally and go nowhere.

The good news is, a rebuilt MC is pretty cheap and the rebuild kits are even cheaper. It's about an hour job to R&R the assembly, bleed and adjust, about an hour longer to do a rebuild.

Good luck, hope this helped in some way.

Mike