Between Omaha and Des Moines, 7 miles South of I80 | I haven't worked on a CAT C-12, in a military vehicle, but I'm surprised that CAT is using the "old style" fuel return valve setup, in a 2007 engine.
Heck, years ago, when I tried to purchase an older style fuel return base, for a truck engine, it automatically updated to the "newer style" design
Have you tried using 2 people, when cranking? = one to operate hand priming pump, constantly, the other to operate the key switch, while cranking.
I would probably change the fuel secondary filter, prime, and probably move the your fuel pressure gauge to AFTER the secondary fuel filter (Then you should be "seeing" pressure inside C-12 fuel rail area)I don't ever expect that old design, of the fuel return line pressure spring/valve to "hold pressure" like the newer design will = the old design is going to bled off and leak down somewhat quickly.
Note: On some older "truck engine" fuel return valve design, if a person used the "bleed plug" may times, there was an internal O-ring that could cut and split, and let fuel flow right pass, where an internal O-ring should have been, thus lower fuel pressure, than normal and "dropped priming pressure" very quickly = I'm not familiar with your particular base, so I don't know if this applies to that base, or not.
 (129-2878 base_in paint program (full).jpg)
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Cat C12 fuel system diagnosis
