What is best to put on threads of the plug to the master brake cylinder

[chevyguyase]

I wouldn't worry about the O-ring. Just suck out about 1/2 inch of fluid. I figure 1/2 inch because the top plate and threaded portion is probably 1/2 inch thick and you want at least 1/4 inch of air between the top plate and the fluid. I think you are putting to much fluid in it. It has to have air space in the master cylinder. No fluid should be near that lid. As long as it keeps dirt out and returns fluid splashed up while you're driving you're good to go. There is no pressure on that lid, it has to vent. Have fun my friend.