The o-ring between the master cylinder and brake booster provides a vacuum seal for the booster; it is not a factor if your master cylinder is leaking fluid. There are also seals (grommets) where the fluid reservoir seats into the master cylinder. They can be replaced if they are the cause of the leak. If the leak is from the master cylinder itself, then it is time for a new master cylinder. Some master cylinders have a small "leak hole" just forward of the flange. If it is leaking there, you have an internal leak in the MC.