Just saw your pics...CLEAN TRUCK !
That metal fuel pipe, pump to fuel filter is awfully close to both heater hoses and could be incurring heat soak from the water hoses...
Wrap the fuel pipe with some heat wrap and maybe try and move it away from the heater hoses...
Also...how old is that mechanical pump? I have seen the valve sets inside the pump work ok when cold, but drain back when hot...
Have fun...
