Wes you will find two seals between the servo and the master cylinder.

1, A small "O" ring around the base of the master cylinder, THIS IS NOT A SEAL, ITS THERE TO STOP DIRT INGRESS ONLY, there is a small notch in the bottom of the master cylinder to release any lost fluid, because of this the master cylinder should not be sealed to the servo.

2, a larger seal just within the servo. this is about 35mm in diameter and seals the servo from air loss.

Matt.