in this case, it can go either way. i would probably say the main relay is the culprit only because it seems like virtually all hondas have had the main relay go out on them... a bad check valve just means the fuel lines between the fuel rail to the fuel pump isn't holding pressure w/ the engine off, but the fuel pump itself will still work. if you had tried cranking the engine for 5 seconds or more and it will still not start, i don't believe you have a bad check valve.