Have you tried replacing the battery in the F3 with a new one? In a lot of the equipment I have, a fading Liquid Crystal Display (LCD) often is an indication of a problem with the power source. Before replacing the LCD, try a new battery. The F3 can probably use a new one every once in a while.
I agree and turn the power switch off and on a few times to wipe the contacts. If its still not bright enough just get another finder.