When you're replacing a clutch in your generation 1 MINI Cooper S, it's best to have all the parts on hand to do the job right and get the car back on the road. A part that we see failing from time to time is the slave cylinder. It can be a 50/50 chance that it's going to be good. Even tho you didn't have a problem to start with, just by taking the part off, it failed and damaged a seal inside of it. There unfortunately is no way around this and you didn't do anything wrong. You're not going to just run down to your local parts store and pick one up, is a very specialized item for the MINI. You will also need some brake fluid and a pressure bleeder, as well as a special tool as seen in the above photos, to do the job correctly. You can make a tool or buy one, but at this time we do not sell or rent them on line, but you could give us a call as we can get them. Basically the cylinder needs to be pressed in when pressure bleeding to get all air out.
Note: this Part is a MINI OEM part in a aftermarket box.