i had to have my slave cylinder replaced, easy to diagnose with a cheap bleed kit.-
you bleed the clutch, to make sure its not "an airlock", you pump the the pedal a few times .... it goes back to being light. Is it the master cylinder or slave? Well if its the master , you should be able to keep pumping it and the fluid level wont change or you will spot the loss of fluid leaking down the bulkhead.... pray this is the case.... If you are losing fluid , i.e. the level is dropping in the reservoir but you cant find out where its going... then get your credit card out , find someone who can fix seats at cheaper labour rates than seat dealers and ask him to put a new slave cylinder in for you ... do the rest of the clutch at the same time. p.s. not one for the home mechanic unless you have lots of gear (trolley jacks , engine hoists etc)., patience ,ability and enjoy a real challenge / or if insanity runs in your family.