heck of a time bleeding brakes.

[gas man]

I have a 39 mercury with a new original style master cylinder, I also replaced all the brake lines, wheel cylinders, and brake shoes. so now I'm on to bleeding the brakes. what I'm having a hard time understanding is do I fill the master from the little 3/8 plug ? or can I pull that bigger looking plug with about a 1 1/8 socket? Also how do you know when your getting low on fluid in the resevior since you cant see it? I'm paranoid and adding a tiny bit of fluid after every time I loosen the bleed screw. I dont want to suck air back in.