If your fuel pressure is too low or is bleeding down when the engine is off, remove the intake tuning valve on the top of the plenum and look down inside the intake plenum with a flashlight. If you can see any signs of liquid fuel or clean spots inside the plenum, you probably have either a faulty/ leaking fuel injector assembly, or a leak in the fuel line assembly that runs from the wall of the intake to the injector. These are both very common failures on this engine.
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Normal fuel system pressure on this vehicle is 66 PSI; if you are only getting 40 psi pressure then you either have a defective fuel pressure regulator or a defective fuel pump. It is the job of the pump to build the pressure in the first place and the job of the pressure regulator to bleed off any excess pressure. THere are no other possible components that can fail to cause low fuel pressure, as l;ong as there are no leaks in the system.
You can check the regulator by going back to the tank area and using a pair of vise grips to pinch shut the fuel return line coming back to the tank from the regulator: if you cycle the key to run the pump and the pressure is still low with the return line pinched shut then the only possible cause is a faulty fuel pump. The pump should be able to build well in excess of the normal operating pressure of the engine.
It is always possible to get a defective new fuel pump, especially if it came from a discount parts store like Autozone, Advance, or Pep Boys... Discount stores buy the absolute cheapest parts they can find since they compete on price alone; often this results in poor quality parts that do not meet manufacturer's specs. If you look around you will find that very few professionals buy their parts there, even though they are cheaper. After you get several defective new parts from discount suppliers you just learn that the wasted labor and frustration is not worth using discount parts.
I would suggest trying a Delco fuel pump assembly; in our shop we have only had one or 2 defective Delco pumps for GM vehicles out of the last several hundred we have installed. Some aftermarket brands we tried had nearly a 100% failure rate...