What is the "bleed off" that you referred to? Are you talking about the fuel pressure test port, which is a schrader valve? You might want to hook up a fuel pressure gauge to that test port and see what pressure it reads with the key turned to on.
The fuel gauge sending unit is simply that. It is connected to a float in the gas tank that moves an arm and changes the resistance that is sent to the PCM and then PCM sends a signal to the fuel gauge. Fuel gauge sending units go bad in ZJs all the time. Mine is bad now.
Apparently, it also is used for OBD II emission monitoring requirements. It is to "prevent the OBD II system from recording/setting false misfire and fuel system monitor trouble codes." (from FSM) "the feature is activated if the fuel level in the tank is less than approximately 15 percent of its rated capacity. If equipped with a leak detection pump (EVAP system monitor), this feature will also be activated if the fuel level in the tank is more than approximately 85 percent of its rated capacity."
I am not sure what all of that means, but I do know that the PCM also controls the electrical pulse delivered to the fuel injectors. The FSM only says that the "PCM determines injector pulse width based on various inputs."
Maybe some of the more experienced members will be able to shed more light...