After starting the car, pull the vacuum hose on the front of the fuel pressure regulator and check for gas and/or an overwhelming smell of unburned fuel. I do doubt that is your problem as the FPR normally gives some warning before it croaks altogether. Has the car been roughing rough or stalling out for awhile?

The check valve screws into the outlet end of the main fuel pump. You did say you replaced your main pump?

Quick and dirty test would be to let the car sit overnight and then loosen the connection from the rubber fuel line to the fuel rail. Have a few rags ready and wear some goggles. If you don't get any fuel squirting out under pressure (just a drip or two) when the fitting is loosened, you could have a bad check valve.

Have you checked or replaced your fuel pump relay recently?