There are 4 possible causes for the fuel pressure dropping to "0" on shutoff:

-faulty check valve in the fuel pump
-faulty fuel pressure regulator
-leaking fuel injector(s)
-leaking fuel lines between the pump and the regulator, including the connections at the pump/bucket

You have to systematically rule each possibility out. Pull the rails, pressure the system, look for drips from the injectors. Pull the FPR vacuum line, and look for wet fuel. Block off the return line, pressure the system. Look/sniff for leaks.