2250 Low Fuel Rail Pressure

[Knuckles]

One of my deman CM2250 occasionally gets low fuel rail pressure code, It wasn't staying primed so i checked for leaks on suction side and installed a new lift pump, ran a high fuel pressure test and as soon as the engine comes to a stop the pressure Immediately goes from 29,000 to 0psi or even -20psi, I changed the check valve, no luck, I isolated each injector with a fuel rail plug and no luck either, I didn't want to buy 5 plugs like quick serve says because they're $108 a piece here, so its possible that more than one injector is leaking but when i do a cylinder cut out test, each cylinder sounds the same. I have removed the pump head it has a two barrel steel plunger, I tested it like quick serve says it looks good, so at this point I'm not sure as to where to look for pressure loss, the oem filter doesn't ever fill to view like it used to so I'm thinking its still getting air suction but I'm not sure how that interferes with the fuel rail pressure drop, any suggestions would be much appreciated, thanks in advance!