i think it goes back to the story you've been telling up to that point.

personally, i really hate the 1/3 river bet simply because it seems to rarely have fold equity when bluffing.

as for the OP, yes, i often bet full pot on the end if i backdoor a flush simply because people don't believe it. that being said, i bet full pot other times, too (a bit different in plo8 due to the split pot nature), and my "value bets" are often 2/3 pot or larger.

Yeah, I'm generally a half-potter on the river whether value bet or bluff, but I noticed at the 1/2 that quite a few players are more full-potters (also on both).

I don't think I'd ever go with 1/3 pot on a bluff, but half-pot can be worth a try sometimes. I do think it's rather dodgy bluffing a backdoor, although I've seen people try it.