The fitting that screws into the slave cylinder should seal on the tapered seat at the bottom of the hole. Check for anything in there that might be making a leak. They do make small, soft, cone shaped washers to put in there that will create a seal. If you are using a fitting that seals on the flat surface on the outside there should be a soft copper crush washer there to make the seal. Make sure that the flat surface of the cylinder doesn't have any nicks to make a leak.