Depends on where you are testing the pressure. If you are testing at the fuel rail, pinch the feed and return line then shut off the pump. If it still drops right off then it its an injector. If not, just pinch the return line and try it, if it drops off then its the check valve in the pump..
BTW The pressure will drop if the return line is not pinched and this is normal.
Sorry to jump the gun..
Figured you were doing an actual test with the return side pinched..
Indications of injector leak down include "hot soak" starting issues (starts like its flooded).
An indication of a fuel pump issue is it cranks for a while like restarting after running out of gas..
If you have neither one then its just the normal pressure down through the return side..
Odds are the pressure regulator is gone in the pump....mine did the same thing...key off...gage dropped instanly to zero...i had the crank to start prob too...
new pump...my gage will hold pressure for couple hours...starts instantly...