Excessive vacuum in the fuel tank.
My dad has had a problem with excessive vacuum in the fuel tank for a little while now (1997 chevy k1500, 350), he has changed most of the EVAP system components and says he changed the fuel cap to a vented one. I went out to his house to help him this weekend because the truck wouldn’t start. The fuel pressure was low and we were removing the fuel pump and I found that the fuel tank was sucked in. There is no external vent for the fuel tank and I suspect that the gas cap he got does not vent. My question is what is this component in the photo? This is the line that runs to the EVAP canister, is this a vapor separator, the tank vent or just a valve used to check the vacuum in the tank? I need to know if we need to replace the fuel sender assembly as well if this is going to cause a problem. Also how would you check this component? Thanks.