this simple diagram may help, you can always google images of "ford idi fuel injection system" or something of that nature & you'll see all kinds of interesting pics.
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I'm not sure what application that diagram is for, but it is not accurate for 7.3 trucks... the separator, heater, and filter are all one assembly.
I also don't think the OP adequately addressed the root issue here.... a leak at the filter head will still allow the engine to run. Since a new filter was tried (and hopefully prefilled) I suppose that eliminates a blockage concern. My concern, and this is an outside theory, is that the lift pump may be failing, and wasn't able to deliver enough pressure to push enough fuel through the filter element.
Slapping on a new filter head may not solve the problem...
Also curious... why is there a T on the IP return line?