i think that it either is like Rad-x and makes you have slower radiation or you will take less rads for food and drinks.
I can say for certian now, it doesn't work like this. Again, I made sure, and I did see the rad reduction the first time, the sink being 5 instead of 2. But now the next day, it is back to 5. I even drank the water and only got +2 rads before, but now I am getting +5 rads, so there is no slowness in radation at all. Like I said, I had 116 rads, took a sip, now at 121. That is a +5 increase just like if I never had the lead belly perk at all.
So now it's a wasted perk. I don't feel like redoing everything now.

. I don't even know if I have the saved game since I thought it was working, I went over it now. :sadvaultboy:
*edit* I did find a saved game. Now I have to redo a quest now. Since Lead Belly is broken at least for me, I might as well as take the toughness perk. Going to switch very easy and just redo everything now.
So this is how the game makes us redo things. LOL. Since we reload, I guess this is the 360's way of reloading the universe LOL.