To correctly check the relief valve you have to get a fuel system test fitting available from glacier diesel I think the number is 9013, but ask Richard you then take the banjo out of the valve and put the test fitting in and start truck if any fuel comes out the valve is bad, theoretically you could just take the banjo out and pull the line to the side that has the cp3 return fuel flow but that's a PITA and it's hard to tell if the fuel is coming from the line or from the relief valve the test fitting is only 20 dollars, I did it about 3 weeks ago. Jay