w123 Clutch issues, mystical fluid loss! Help me before it tear into the wrong thing.

[arcticathlon]

W123, 1980, 240D, standard.

2 months ago i noticed that the clutch was not as firm and acting different. checked the fluid, and fluid was low. refilled fluid, checked for leaking under car and at all wheels and found nothing. re-primed the clutch using the passenger caliper and it drove great for a day (20 miles).

Next day on my way to work after 2 miles i feel it acting funny again and turn around. by the time i got home it was almost to the floor to shift. park the car, check for leaks and cant find any, and so I research some. order Master Brake cylinder, as other said that dissapearing brake fluid could be from a bad master cylinder that is being ingested by the engine via the vacuum line...

New master cylinder arrives and I wait a month before i get the chance to replace it (this is today). i go outside and was going to pull it up a little in the driveway, but when it press the clutch to start it, it has ZERO fluid resistance and just plops all the way to the firewall!!! the spring pulled it back completely by itself, after a slight bump. The brakes still work fine with the engine off and i tested them by rolling down my driveway and stopping. The brake reservoir is half full and drained to the Min fill line, which is where i think the clutch supply pulls from.

So, now i am wondering if i really should not crack open my Brake system and instead look at my clutch hydraulic system. about 1 year ago i replaced the clutch slave cylinder with a generic (possible lesson learned...)

The odd thing is that i don't know how the clutch would go completely empty by just sitting. Is that the symptom of a Clutch Master or Clutch Slave? Also, seems unlikely that it is the Master Brake. where did my fluid go? slow leak that I did not notice??

any input is appreciated.