It fills through the dipstick hole. Any leak should be of concern, if not just for a possible safety hazard. If it is leaking from the control valve its a pretty easy fix. If it is at the housing seams or torque converter seal it will be pretty costly to fix. I also am assuming this is an older Yale, the Veracitor series also has a drive shaft seal that can leak and also does not use ATF as a lubricant.