Having problems starting my 500efi's after running them hard than setting for several hours. Fuel system not priming itself, had to fill fuel separater to get engines to start.

Any thoughts? Fuel tanks full.

Either the fuel pressure regulator leaking down or the fuel pump check valve is history. Fuel pressure leaking back to the tank. I'd bet on the fuel pump since it won't reprime. If the reg is leaking it will usually just cause a long crank but will restart.