Gas Tank Relocating Filler Question

[tripleblack]

I noticed that there are many places to place your filler for a rear gas tank conversion. I know about the side filler(along the rear fender), i have seen someone who placed it behind their tail light( which i might add was the coolest thing i have ever seen), and i have seen the Bed flush filler. I was wondering if you had the bed flush filler, which i think is the easiest, if it was a way for water from rain to leak into your tank? I know the cap is supposed to make an air tight seal but you know that isnt always the case. Thanks for your ideas and input.