1995 Ford F350 Fuel leak

I'm out of ideas for what may still be causing a slow disel fuel leak. I have replaced the fuel pump and hoses, added new metal washers at the banjo fitting and noticed a small leak at the vacuum sensor (fuel restriction sensor) and replaced that and still am seeing disel in the valley but cannot visable see fuel leaking even with mirrior and flashlight. Should I place a new gasket at the fpr? I can't see from any angel but should I assume the drain valve is leaking from below? The fuel fillter canister appears to be OK.