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i found no restrictions. except for that check valve being bad. what all do i need to take off to get the old pump off?

no sir the check valve is allowing fuel to slowly bleed down out of the valve. i will check the primary side now. 

the line on the front of the engine is where i put my gauge and it never moved. i removed the return line and primed my hand pump and check valve had pressure on it. so i know i need to replace the return line.

yes sir it is the grey engine. chn613. the line that goes above the compressor is dryrotted. is that my suction line if so i believe i should start with that correct?

i have an 06 Mack and i was wondering why i have lost all fuel prime.  i put a gauge on it and it doesn't move. whenever you try to start the truck it will not start. but if i prime the pump while someone else is turning the key i may get lucky and it may fire up. as soon as you shut it down, you cannot start it back up. my real question is. do i have to pull the front of the engine to replace this pump? also which lines move the fuel from the tank to the fuel rail and which move fuel back to the tanks