Fuel pressure test

[SoAlWS6]

I am trying to test my fuel pressure on my 2002 Trans Am with ls1. I checked it at the Schrader valve and got 54 PSI but when the pump shuts off it drops to zero. I am trying to isolate the injectors to see if the check valve is bad in the fuel pump and not a leaking injector. If I disconnect the quick connect at the fuel line coming into the fuel rail I could hook my fuel pressure gauge there and check the pressure bypassing the injectors. That would verify if I have a bad check valve. I need a 3/8" quick connect adapter that I could use to do this with. My fuel pressure test gauge has the Schrader valve fitting on it, I don't know the thread size.
What is the thread size on the fuel rail Schrader valve fitting?
I found a 6an male flare to 3/8" quick connect, it does not fit my gauge fitting.

[SoAlWS6]

Got a 4an male to to 6an female adapter, will allow my 6an to 3/8" male quick connect at the fuel rail supply.