Replace the whole actuator. I tried to replace just the master cylinder and had a pain in the butt time with it. I took it back, bought the whole actuator for just a little more money and it was an ease to put on. Be careful how much torque you put on the bolts that hold it to the trailer tongue. You can cave in the tongue. Don't ask how I know.

Good luck, Wally "FishnFed"