To test if it's your Hydraulic head(o-rings or shaft), simply remove the shut-off cover and turn on your in-tank pump. Look with a mirror and you'll know for sure if the leak is there or not.
FDC, either bypass it(as the military did) or lift it out(4 bolts) and turn on your in-tank pump, and watch for leaks. Bypassing it is really the best plan.
Fuel fired intake heater, easily rebuilt with a couple O-rings.