Fuel tank o-ring tip: (I know it works on my NEST, but I don't know about other fuel tanks)

The other day the o-ring was not sealing properly on my tank. Instead of replacing the o-ring... try to tighten down the screw on the underside of the cap. This will compress the o-ring and cause it to increase the width of the o-ring... which in turn tightens the fit. I tightened my screw 2 full turns over 2 months ago and my tank hasn't spilled any fuel out of the top and it has maintained the proper seal ever since.