In this sentence, I think "at" is best. However it sounds a bit clumsy to me but OK for a caption. How about "...seat and plug when the valve is 100% open."" You could also say "fully open".
If you used "on" you would imply that that the valve has just opened rather than just being open, but it wouldn't really work in this sentence
"in" doesn't work here. An example of how it could be used "in cases where the valve is 100% open".