Paccar fuel filter doesn't stay full

[Rawze]

Have you pressurized the fuel tanks to 10 psi and let it sit a few hours to check for leaks?.

WARNING: ANYONE WHO PRESSURIZES THE FUEL TANKS FOR TROUBLESHOOTING PURPOSES ... NEVER OPEN THE FUEL CAPS WHILE THE TANK IS PRESSURIZED!!!! .. NOT EVEN A FEW PSI!!! .. BECAUSE THE FUEL CAPS CAN, AND WILL BREAK YOUR HAND OR ARM IF YOU TRY TO REMOVE THEM!!!, OR put a person in the hospital real fast!. There is a LOT of force behind those fuel caps even with only a very few psi pressure in the fuel tanks. Also make sure there are no flame sources around due to possible fumes.

I have also seen cracked welds at the top of the pick-up tubes inside the fuel tanks on a few trucks that allow air in too. Putting a fuel line into the bottom of a bucket and using air pressure to push the fuel out of the tank will show bubbles mixed with the fuel to reveal this.

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If it were here, would be interesting to see for myself the difference in micron ratings and to help find a possible cause.