Losing prime overnight/fuel pressure problem

[tree98]

You must be getting a check engine light? Probably a 0559 for low rail pressure right? You need to get it to run and pass that leakdown test properly.

You mentioned loosening the relief valve on the end of the rail while it was pressurized........

DO NOT LOOSEN FITTINGS ON THAT RAIL WHEN ITS PRESSURIZED!!! THERE'S ONE SPECIFIC FITTING YOU CAN LOOSEN, YOU'LL FIND IT ON QUICKSERVE. THAT LEVEL OF PRESSURE CAN SERIOUSLY HURT YOU!!!

Did you take the pump head off and inspect for damage?

Replace the relief valve if it's the original.

Replace all the guts in the high pressure pump if it has 400k or more miles on it.

Do you have the injector caps for testing injector leakage? If not go buy them and start capping injectors 3 at a time.