dropping fuel pressure after shutdown

["Dave Aukerman" <mhauk@ccsdana.net>]

89 200tqw is dropping fuel pressure after shutdown (stopping at 5psi after
45sec).  Fuel pump check valve is good according to Bentley test.  Running
fuel pressure is ~55-57psi and upon shutdown it heads straight for the
basement.  I have switched out the following: fuel pres. regulator, fuel
pres. controller, cold and warm start valve and fuel distributor.  Fuel is
not running out of the injectors.  I am running out of options and fuel
system parts.  Vac. and ignition both are fine.

Help would be greatly appreciated as I am getting tired of driving the ole
Ford truck everywhere.

Dave