There is, what I suspect is a non return valve ((NRV )in the fuel system. It is on a little wobbly bracket, held with one screw to the wooden frame behind the passenger. If you flick it with your finger it rattles. Mine does. I posted a video of it on here some time ago but no one else had an issue.
Perhaps this is your problem. It did take me a while to locate it as it sounds more serious. The video is still on my blog if it helps.
http://www.westmog.wordpress.comGood luck
Robert
