you need to find where the the fluid is going. The hydraulics are a closed system meaning you shouldnt have to refill the reservoir. That hydraulic fluid is going somewhere and thats not good. It could be a master or slave cylinder. could be a break in a line somewhere. probably a few other things I don't know.
Anyway, if you are refilling it, the fluid goes somewhere. where is it going? Look for leaks. I'm pretty sure the slave cylinder requires dropping the trans for repair/replacement.