Brake fluid tends to dissolve a lot of things. I would get whatever sealer you used out of there before it becomes a sludge that moves downstream and really gives you problems.
There is a sight glass on the reservoir. Just put enough fluid in so it is visible in the sight glass - you don't need to fill it. You will need to keep an eye on it as the brake pads wear, and add a bit if needed, but it doesn't need to be filled to the brim for the brakes to work fine.
As mentioned, keep the diaphragm and the mating surfaces dry and don't gorilla the cover screws. Even and snug...
