Even if the reservoir level goes below the clutch bleed port, if there are no leaks in the clutch hydraulics it will still work and continue to work. You most likely have a leak.

I find it quite fascinating that someone could diagnose this problem as the slave cylinder without having seen the car or gotten further information. Quite impressive.

Before I effected repairs based on this long distance diagnosis, I would investigate for myself. Begin by seeing if there is any brake fluid on or under the drivers side floor mat. If so, the problem is most likely the master cylinder. If not, the slave cylinder should be checked next. Pull it out of it's mount and see if any fluid has made it past the piston.

Also, when checking the master cylinder, ensure that the hose has not slipped off the top.