07 fuel lines

I have a 07, my fuel line started leaking. Went to replace and it seems the line from tank is a suppose to be 3/8, but went to replace fitting in nylon hose but the 3/8 connector was to big, when I put fitting to hose it was larger than hose, a 5/16 was also a very tight fit even after heating tube up. The problem is that I measured line up top going near fuel rail and it measured 9mm(.345 inch) and 5/16(.312 inch) has anyone run into this problem? Also looked at a diagram for fuel lines and it shows 3 lines from tank and I only seem to have 2. Any solutions? Thanx

There should be a pressure line, a return line, and an EVAP line from the tank to the engine compartment.

You will have to be more specific about the fitment problems. The nylon is necessarily smaller than the barbed fittings designed for it, and the hose needs to be controlled heated and accurately pressed onto the barb. They are not hand-assembly fittings.

Alternately, the hard line ends of those fittings should be very close to the nominal line sizes since they rely on an O-ring for sealing. If the size mismatch is there, a 3/8" fitting will not fit a 10mm line, and vice-versa.

2003 Trailblazer fuel plumbing, attached.

Two hoses attached to the fuel pump assembly, one attached to a separate valve farther back on the tank.