Low fuel light question

Hey all,

I just replaced all of my instrument lights and idiot lights, and now my low fuel light works... but not correctly. When I turn the key on, it flashes on and off in a fraction of a second, while the other idiot lights for brake, ABS, wait to start, and water in fuel stay on for a couple seconds. I am guessing that is normal, as I have seen youtube videos of other people's and it does the same. But if I leave the key in the "on" position or crank the truck up, the low fuel light stays on, but really dim. You can just barely see it in the day time, if you look close. I have just over half a tank of diesel, so it shouldn't be on, and the gauge works accurately. What could cause this? When it does the self test, it is much brighter, like all of the other idiot lights. Thanks!