there is a check valve originally on top of the fuel pump inside the tank, if that fails, all fuel goes back into the tank, Bill R. shows here how to install a check valve on top of the tank
http://www.bimmernut.com/forum/5-ser...d-535-her.html
You can use any fuel check valve with that diameter which can stand at least 3-4 bar. Usually the shops which handle bio fuel parts have them.

Have you checked if the fuel pump is loose inside the tank and maybe floating?